https://metacpan.org/pod/Dancer%3a%3aDeployment#Running-from-Apache-under-appdir (i know not dancer2 ) warns of Plack::Handler::Apache2 issues
NOTE: Only a single Dancer application can be deployed using the Plack::Handler::Apache2 method. Multiple Dancer applications will not work properly (The routes will be mixed-up between the applications). It's recommended to start each app with plackup using your favorite server (Starman, for example) and then put a web server (Apache, Nginx, Perlbal, etc.) as a frontend server for both apps using reverse proxy (HTTP based, no fastcgi).
both warnings should have linked to a test case, as both bugs are dumb