I have been using XRDP for a long time on all my systems which include Leap 15.1 and Tumbleweed. Not has any issues but each connection opens up a new session, as expected. Documentation suggests it is possible to connect to the existing main console session. Can’t find any references to this with SUSE but checking some Ubuntu forums it looks like adding this code to xrdp.ini should work.
Every flavor of VNC is set up and configured somewhat differently, so when you read general guides on the Internet, you have to be very discerning or strictly follow TigerVNC documentation (which is what openSUSE uses).
For your specific question about connecting to an existing session…
The following describes how to set this up
I am not actually running a VNC server but XRDP is using libvnc. It maybe the case that it is not possible on SUSE, hence the request for anyone that has made it work.
Then you’ll have to describe how you’re using XRDP, it’s only a protocol and <how> you use XRDP in an application might make a diff…
So, for instance I use XRDP when I forward X over SSH, I often fine it convenient to set up by using the YaST module to set up and configure VNC which also enables problem-free use of X over SSH.