Hi,
I am on debian 10 with hestia 1.4.17 running. (apache2 + nginx)
I know for sure that webmail.domain.xyz worked a few weeks ago (unfortunately I use webmail rarely so I cannot tell when that changed).
DNS is correct and as said - it surely already worked but yesterday I had to use webmail again and discovered that the webmail only shows the source of the index.php file.
I than tried to switch between rainloop and roundcube (both showed their respective index.php as source)
As I found in another thread where that happend that also phpmyadmin and phppgadmin were not using php I tested this too, but to my surprise phpmyadmin and phppgadmin were correctly showing up.
So it is only webmail which obviously does not use php to process the page source.