I followed your instructions to use the Time Remap filter to freeze the last frame of a clip. I tested in both 24.08.29 (latest version) and 24.06.26 on Windows 11. The instructions worked perfectly for me in both versions.
Your problem sounds similar to this problem reported previously by another user.
Unfortunately, I have not been able to reproduce that problem. And if I can not reproduce it, I can’t fix it. I wonder if it is related to some difference in system configuration like language or locale.
As a workaround, you could make your time remapping edits in another instance of Shotcut and then export an intermediate file to import into your main project. Similar to the work-around you are doing today.