We cannot reproduce this issue. Can you provide your exact steps for reproducing this problem starting with an image with no metadata? If you can provide screenshots to illustrate or make a screen recording where you reproduce the problem, that may be helpful.
I ingest photos using PM. During the ingest all photos get a basic metadata (including the original file name stored in intellectual genre field and "my filename" stored in multiple fields like Special instructions, Job ID, Edit Status and IPTC Subject code.
Some times I ingest multiple events' photos from the same card, so I have to overwrite the captions and other info, but I want to keep the original filename and my filename infos as they belong to the image file not to the event.
In these cases whenever I try to overwrite the other IPTC data in the files but keep the above mentioned fields untouched, I keep experiencing the very same.
I have some screenshots, and an example file. I send them to you in a personal message.