Hi.
# Opt in Timeless.
# Open some page without categories.
# Click on wikitext editor.
# Add manually some category A.
# Save.
# The category A missing.
# Click on wikitext editor.
# Add manually some category B.
# Save.
# The category A appears, the category B missing.
# Click on wikitext editor.
# Make a nulledit.
# Both categories appear.
# Click on wikitext editor.
# Remove manually category A.
# Save.
# Both categories appear.
# Click on wikitext editor.
# Remove manually category B.
# Save.
# Category A removed, category B appears.
# Click on wikitext editor.
# Make a nulledit.
# Both categories removed.
It can be a need for phisical extra edit, or a need for rebuild, or waiting for task queue update.
Thank you.
See also T175713, T175714.