Audio tracks are dropping after import and after working ok - can't replace footage

Hi,

I import four mp3 stems and place them in my timeline. They all work ok. When I exit out of Pr and come back in, Pr has dropped one of the stems (greyed out missing footage). The other three stems in the same folder are unaffected. I can't 'replace footage' by locating the file from the same directory. Instead, I have the place the same file in a new folder and 'replace footage' that way. There is nothing wrong with these files, they play fine on first import, fine in other audio apps, and fine once replaced from a different location.

When I exit Pr again, it does the same thing. Now I can't 'replace footage' from either of those two locations and have to put it in a third folder. And so it continues...

I have deleted my media cache directory. I'm running Pr 23.1.0 (build 86). NVIDEA Studio Graphics driver is up to date (528.24). This problem first started a couple of versions ago.