There is an issue with the duplicate image function (command-D). When you make a duplicate of an image, the second image does not appear. You have to quit PL3 and restart, and the duplicate image is there. Or multiple images are there, if you did it four or five times wondering what happened.
This issue was a problem back in PL2. It was eventually fixed and I didn’t see it for a long time. It appeared in one of the recent builds of PL3, then it disappeared, but now it’s back again.
Actually this is not resolved, but hard to reproduce unless you know the circumstances. See PhotoLab 3.1.0.27 Bug! Virtual copies do not update film strip
The issue is reproducible, also in 3.1.0.29 on MacOS, if you start Photolab by “Open with…” from Finder. I had the same in PL1 also.