Thank you so much, @cgrenier and @recuperation!
It is version PhotoRec 7.2-WIP, Data Recovery Utility, February 2023, on a 64-bit Windows 10 machine. This phenomenon only occurs for this one specific drive, for all other drives I had used before with PhotoRec, the process always finishes with the “Recovery completed.” screen.
The drive has around 625 000 000 sectors (320 GB). Yes, PhotoRec has recovered a lot (thousands) of files, probably all that were possible. I can't really tell if PhotoRec skips anything at the end or whether it actually reaches the end (It counts too quickly for my eyes. One of the last numbers I could read before the progress stops is something around sector 624 500 000), but I can tell that it does not show the "Recovery completed" screen when recovering that specific drive and I have repeated that procedure out of curiosity a few times. That's really interesting that it suddenly shows the source partition selection screen insted of the "Recovery completed" screen.
In the photorec.ses file, the lines after the line ending by ",inter" are the sector range that PhotoRec need to parse to recover the remaining files.
How is it possible that the sector range has numbers like 123 456 789 012 (12 digits) when the entire disk only has 625 000 000 (9 digits) sectors?
Thank you very much and have a nice day!