Usar subconsulta em WHERE torna a consulta extremamente lenta

Eu tenho essa consulta bastante básica que é muito lenta por motivos que não consigo descobrir:

SELECT s.id 
FROM segments s
WHERE
    ST_DWithin(
        s.geom::GEOGRAPHY,
        ST_Envelope((SELECT ST_COLLECT(s2.geom) FROM segments s2 WHERE s2.id IN (407820025,  407820024,  407817407,  407817408,  407816908,  407816909,  407817413,  407817414,  407817409,  407817410,  407817405,  407817406,  407816905,  407816907,  407817412,  407817411,  407816906,  407816904,  407816764,  407816765)))::GEOGRAPHY,
        30
    );

                                                                                                                           QUERY PLAN                                                                                                                            
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on segments s  (cost=55.58..48476381.06 rows=7444984 width=4)
   Filter: st_dwithin((geom)::geography, (st_astext(st_envelope($0)))::geography, '30'::double precision)
   InitPlan 1 (returns $0)
     ->  Aggregate  (cost=55.57..55.58 rows=1 width=32)
           ->  Index Scan using segments_pkey on segments s2  (cost=0.44..55.52 rows=20 width=113)
                 Index Cond: (id = ANY ('{407820025,407820024,407817407,407817408,407816908,407816909,407817413,407817414,407817409,407817410,407817405,407817406,407816905,407816907,407817412,407817411,407816906,407816904,407816764,407816765}'::integer[]))

Onde estou realmente confuso é que o ST_Envelope com a subconsulta é muito rápido por si só

SELECT ST_Envelope((SELECT ST_COLLECT(geom) FROM segments WHERE id IN (407820025,  407820024,  407817407,  407817408,  407816908,  407816909,  407817413,  407817414,  407817409,  407817410,  407817405,  407817406,  407816905,  407816907,  407817412,  407817411,  407816906,  407816904,  407816764,  407816765)))::GEOGRAPHY;

                                                                                                                           QUERY PLAN                                                                                                                            
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=55.58..55.60 rows=1 width=32)
   InitPlan 1 (returns $0)
     ->  Aggregate  (cost=55.57..55.58 rows=1 width=32)
           ->  Index Scan using segments_pkey on segments  (cost=0.44..55.52 rows=20 width=113)
                 Index Cond: (id = ANY ('{407820025,407820024,407817407,407817408,407816908,407816909,407817413,407817414,407817409,407817410,407817405,407817406,407816905,407816907,407817412,407817411,407816906,407816904,407816764,407816765}'::integer[]))

E assim é a consulta principal se eu plugar o resultado do ST_Envelope

SELECT id 
FROM segments
WHERE
    st_dwithin(
        geom::geography,
        '0103000020E61000000100000005000000C87B6E0D8FB85EC04BFD8462B9C34640C87B6E0D8FB85EC0929B35C16DC44640BBF8DDA6F2B75EC0929B35C16DC44640BBF8DDA6F2B75EC04BFD8462B9C34640C87B6E0D8FB85EC04BFD8462B9C34640'::GEOGRAPHY,
        30
    );

                                                                                                                                                                                                                                                                                QUERY PLAN                                                                                                                                                                                                                                                                                
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using segments_geom_geo_idx on segments  (cost=0.42..4.82 rows=1 width=4)
   Index Cond: ((geom)::geography && '0103000020E61000000100000005000000C87B6E0D8FB85EC04BFD8462B9C34640C87B6E0D8FB85EC0929B35C16DC44640BBF8DDA6F2B75EC0929B35C16DC44640BBF8DDA6F2B75EC04BFD8462B9C34640C87B6E0D8FB85EC04BFD8462B9C34640'::geography)
   Filter: (('0103000020E61000000100000005000000C87B6E0D8FB85EC04BFD8462B9C34640C87B6E0D8FB85EC0929B35C16DC44640BBF8DDA6F2B75EC0929B35C16DC44640BBF8DDA6F2B75EC04BFD8462B9C34640C87B6E0D8FB85EC04BFD8462B9C34640'::geography && _st_expand((geom)::geography, '30'::double precision)) AND _st_dwithin((geom)::geography, '0103000020E61000000100000005000000C87B6E0D8FB85EC04BFD8462B9C34640C87B6E0D8FB85EC0929B35C16DC44640BBF8DDA6F2B75EC0929B35C16DC44640BBF8DDA6F2B75EC04BFD8462B9C34640C87B6E0D8FB85EC04BFD8462B9C34640'::geography, '30'::double precision, true))

O Postgres não deveria calcular o ST_Envelope uma vez e usá-lo para a condição WHERE, efetivamente fazendo o que eu fiz manualmente? Também não entendo porque nenhum índice é usado para fazer o filtro na consulta original.

Tentei colocar a subconsulta em um CTE, mas isso não resolveu o problema.