Thanks, @RoiDesRois , not exactly, it only happens to special episodes, which are gathered in Season Specials before Season 1, and I just confirmed that to trigger this problem, one does not have to mark multiple special episodes, but there may be some delay, so clearing cache manually between each step may help with reproducing:
Step 1. Mark an episode from Season Special by clicking the purple mark
Step 2. Season Special appears to be half-watched as expected, but just for now
Optional Step. Clear the cache manually
Step 3. The episode appears in History as expected
Step 4. Click that episode in History to open the episode’s page, where the mark is white instead of purple, which is not expected
Step 1. Go back to Season Special at Step 1/2 to find it turning back to Step 1, which is not expected