how does undertow configuration look like?
where is your apache https request endpointed on WildFly? https or http listener?
do you have proxy header forward configured?
I use the default conf from the unzipped WF 18, just for completeness the only modification done is related to the datasources
The apache configuration is shown below
ProxyPass /myapp/ http://wildfly18:8080/myapp/ connectiontimeout=5 timeout=680
ProxyPassReverse /myapp/ http://wildfly18:8080/myapp/
RequestHeader set X-Forwarded-Proto "https"