Yes, I confirm pressing “OK” and the change is not taking place.
The problem looks very much like the one reported by Martin_Be and mentioned by Richard_Lanyon April 10 “Renaming of instruments is ignored. Bug? (Dorico Elements 5.1.30.2109) - #4 by dspreadbury” (Renaming of instruments is ignored. Bug? (Dorico Elements 5.1.30.2109)))
Also, I can confirm that the workaround suggested in the above post also works for me - I just tried it.
Since you don’t have that problem using your program which is at the same update level as mine, I would suspect the update package by which the Steinberg Download Assistant has updated my version.
Hence, is there a way for me, as a user, to completely remove my Dorico program (without loosing license etc.) and re-install it directly at version 5.1.30.2109 (rather than successively upgrading or updating it up to that level - e.g. do like a new user and download from the shop store)?