The issue has recurred. I was able to reproduce the problem this time!
- Run XY. I use a portable installation with a locked pane tab on each side, as noted above.
- Open a new tab on the right side. I now have one tab on the left, and two on the right.
- Configuration -> Preview -> Previewed Formats -> Toggle User-Defined Preview Handlers
- Exit without saving.
- Upon running XY again, the temporary tab is now erroneously displayed along with my usual locked tabs.
I verified that \Panes\2\pane.ini was modified such that "Location=..." now refers to the erroneous tab that should not reappear upon restart.
This is the first time that I have used the UDPH, but the problem has occurred in other contexts as well, and so is more like a problem that occurs when certain settings are changed, or when certain kinds of windows (e.g., preview or file finder) are displayed.