Regras do RewriteEngine ignoradas pelo Apache no SSL

Eu tenho uma configuração Apache funcionando no CentOS 7.6 usando o pacote padrão Apache 2.4.6-88, usando HTTP. Estou tentando habilitar HTTPS no servidor e tudo funciona, exceto minhas regras RewriteEngine. Não consigo encontrar nenhuma menção em nenhum lugar sobre o RewriteEngine não ser suportado pelo mod_ssl.

Habilitei o HTTPS instalando o pacote mod_ssl, e o Apache agora está servindo HTTP e HTTPS da mesma configuração. Há uma VirtualHostdeclaração em /etc/httpd/conf.d/ssl.conf, eu não toquei nisso. A parte relevante da minha configuração se parece com isso:

# Set up Apache proxying
ProxyRequests Off
ProxyPreserveHost Off

# URLs to handle locally
# Apache built-ins
ProxyPass /icons !
ProxyPassReverse /icons !

# Handle local URLs through Apache
RewriteEngine On
RewriteRule ^/$ /index.cgi [L]
RewriteRule ^/favicon.ico /ssg/favicon.cgi [L,R]

ProxyPass /ssg !
ProxyPassReverse /ssg !
ProxyPass /index.cgi !
ProxyPassReverse /index.cgi !
ProxyPass /robots.txt !
ProxyPassReverse /robots.txt !

# Hand off everything else to the Varnish backend, which will in turn
# forward to the appropriate backend server process.
ProxyPass / http://localhost:6081/ retry=0
ProxyPassReverse / http://localhost:6081/

As partes de proxy funcionam, todos os URLs, exceto os listados com o "ProxyPass !" as regras são passadas para o back-end do Varnish.

A parte Rewrite só funciona para HTTP, no entanto. Então http://myip/irá redirecionar para http://myip/index.cgie http://myip/favicon.icopara http://myip/ssg/favicon.cgi. No HTTPS, ele não funciona e, em vez disso, é feito proxy para o back-end do Varnish (que imediatamente relata um erro).

Devo estar perdendo algo óbvio aqui, mas não consigo descobrir o quê.

Habilitando logs muito detalhados, parece ignorar completamente as partes RewriteRule:

[Fri Dec 14 13:38:30.248204 2018] [core:trace5] [pid 90883] protocol.c(647): [client 10.0.30.15:54280] Request received from client: GET / HTTP/1.1
[Fri Dec 14 13:38:30.248282 2018] [ssl:debug] [pid 90883] ssl_engine_kernel.c(225): [client 10.0.30.15:54280] AH02034: Initial (No.1) HTTPS request received for child 21 (server fe80::c9d0:5dd9:c7ed:2045:443)
[Fri Dec 14 13:38:30.248297 2018] [http:trace4] [pid 90883] http_request.c(312): [client 10.0.30.15:54280] Headers received from client:
[Fri Dec 14 13:38:30.248301 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Host: 10.0.28.168
[Fri Dec 14 13:38:30.248303 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Connection: keep-alive
[Fri Dec 14 13:38:30.248305 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Cache-Control: max-age=0
[Fri Dec 14 13:38:30.248308 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Authorization: Digest username=\\"admin\\", realm=\\"Software Activation\\", nonce=\\"redacted\\", uri=\\"/\\", algorithm=MD5, response=\\"redacted\\", qop=auth, nc=0000002d, cnonce=\\"redacted\\"
[Fri Dec 14 13:38:30.248310 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Upgrade-Insecure-Requests: 1
[Fri Dec 14 13:38:30.248312 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.85 Safari/537.36 Vivaldi/2.2.1388.21
[Fri Dec 14 13:38:30.248315 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   DNT: 1
[Fri Dec 14 13:38:30.248316 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
[Fri Dec 14 13:38:30.248328 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Accept-Encoding: gzip, deflate, br
[Fri Dec 14 13:38:30.248383 2018] [http:trace4] [pid 90883] http_request.c(316): [client 10.0.30.15:54280]   Accept-Language: sv,en-US;q=0.9,en;q=0.8,nb;q=0.7
[Fri Dec 14 13:38:30.248513 2018] [authz_core:debug] [pid 90883] mod_authz_core.c(835): [client 10.0.30.15:54280] AH01628: authorization result: granted (no directives)
[Fri Dec 14 13:38:30.248542 2018] [core:trace3] [pid 90883] request.c(304): [client 10.0.30.15:54280] request authorized without authentication by access_checker_ex hook: /
[Fri Dec 14 13:38:30.248577 2018] [proxy_http:trace1] [pid 90883] mod_proxy_http.c(60): [client 10.0.30.15:54280] HTTP: canonicalising URL //localhost:6081/
[Fri Dec 14 13:38:30.248640 2018] [proxy:trace2] [pid 90883] proxy_util.c(1985): [client 10.0.30.15:54280] http: found worker http://localhost:6081/ for http://localhost:6081/
[Fri Dec 14 13:38:30.248657 2018] [proxy:debug] [pid 90883] mod_proxy.c(1123): [client 10.0.30.15:54280] AH01143: Running scheme http handler (attempt 0)