Hello Tobias,
It would be interesting to know which Build Level you are on in Plex 7.2.1. Are you below 24005 ? I am asking because we had a similiar case in WinC environment which could be solved by a fix (2574) contained in Build Level 24005. The fix was at tool level so it could be that it also solve your problem.
So if you are below 24005 you could perhaps give Build Level 24005 a try.
Attention: For Build Level 24005 there were some important on top hiper fixes available which had to be applied on top of Build Level 24005. So if you want to give Build Level 24005 a try to see whether it also solves your problem, have your Plex support partner provide you also with the hiper fixes for Build 24005.
Best regards
Anette-Nicole