Description of problem:
I have a RHEL8.3 workstation with xrdp-0.9.14-3.el8.x86_64 installed.
This provides a login to a desktop for people to run various applications, typically based on "ROOT" and "Geant4" as well as other simulation and analysis tools for high energy physics.

However, whenever a user starts an application that presents a window that has transparent elements, it shows the window (or desktop). This is not supposed to happen, e.g. with Geant4 we end up seeing the text of a terminal below through a transparent object in the OpenGL viewer, and in ROOT's TCanvas many of the lines drawn in graphs or in the general UI (menus etc.) become transparent, making them hard to read.

Version-Release number of selected component (if applicable):
xrdp-0.9.14-3.el8.x86_64

How reproducible:
Every time

Steps to Reproduce:
1. Install xrdp, leave default configuration
2. Login (with Gnome3 / XVnc session)
3. Start such a program, i.e. with root:
$ root
root [0] TH1D* v = new TH1D("","",50,-1,1);
root [1] v->FillRandom("gaus",1000)
root [2] v->Draw()

Actual results:
Background bleeds through windows (not intentionally)

Expected results:
No bleed-through

Additional info:

This can be fixed by a configuration change:
In /etc/xrdp/xrdp.ini, set
max_bpp=24 in section [Globals]
and
xserverbpp=24 in section [Xvnc]

With this, it works as intended.
Submitting a bug report in hope that the default config or underlying issue can be fixed, or so that other having the same problem can find it easilly.

Also, note that connecting with "wrong" bit depth (e.g. in Remmina) results in the connection being closed immediately following acceptance of password. This also happens if changing the config file and restarting xvnc without killing the underlying vnc server process -- service xvnc restart should probably result in killing of vnc server processes?

If wanted, I can configure it back to the buggy state and add a screenshot showing the issue.