Maybe a old bug cropping back up. Awhile back before Plasma 6.3 when I would go to do my defaults to my liking they would default back to what they originally had been. The only way I could get it to behave was remove every other association just leaving the program I wanted the file type to open with.
For as long-standing and intermittently reproduced as that bug has been, there’s likely something tricky about consistently tracking its origins or reproducing it on developer machines. If you’d like, you can at least add yourself to the CC list in Bugzilla to get email notifications whenever new comments are added or the status is changed