Thanks Matt!
Well, I've now installed MC 30.0.33, and copied an album to my desktop, created a new test library, imported those files, and retested (logs attached).
After import & analyze, I started playback (track "x"), and clicked on the progress bar (towards the end). The progress bar is stuttering at the end (jumping forward, definitely noticeable), and ignores other commands. You explained it, and I guess it is what it is, although that's not really good, never saw any other player do that.
Anyhow, the next track starts ("x+1"). I click skip backward, and playback starts not with the previous track ("x"), but one track before that ("x-1"). Skip forward and I'm back at "x+1". Skip backward: "x-2", skip forward: "x+1", skip backward: "x-3", skip forward: "x+1", skip backward: "x-4", skip forward: "x+1". Once skip backward reaches the first track of the album, and I skip forward (again, "x+1"), skip backward will no longer work at all (as there is no track in front of the first track of the album).
So, still the same with MC 30.0.33, and even with local files.