I'm going to be quite frank here. This is not a Pi-hole error. if you completely remove the reverse proxy form the equation pi-hole will function as intended.
That being said if the solution is to merely whitelist the domain within pi-hole why have you not done that using one of the many methods of whitelisting a domain. We have hundreds of users with zero issues running their Pi-hole behind a reverse proxy having 0 issues.
the reverse proxy domain will change with every installation so its not something we can set globally and have work every time. Advanced configs such as this are fully on the user and we expect users who wish to use them to know how they work.