I had 43 channels after doing some clean up. The channels were in no specific order. I wanted to sort them alphabetically the renumber. After I sorted alphabetically, I had 39 channels not 43. In that there is no idiot function (undo) the were no recoverable. Resorting by channel number (original arrangement) still revealed only 39 channels. I don't know how you would duplicate such a thing. Just sorting doesn't seem to change anything.
Well, if you didn't program your radio with the altered file, then you should be able to download your radio again and try to reproduce the issue.
Clicking on the column to sort doesn't change anything in the image itself, it only affects how it's displayed. The sorting is a function of the widget, which is part of the graphics library and not part of CHIRP itself. So, I'm a at a bit of a loss as to how sorting could possibly affect the image and memories themselves.
If you see it again, try to figure out what steps you took to get there so that we can try to figure out what might be going on. Also, noting which memories actually disappeared (like locations 3, 5, and 12, etc) and send me the image.
I bet VK3UR does a lot of sorting with radios packed to the gills with memory locations. Dave, have you ever seen memories fall out of the view when sorting by column?