The bug

All tags that are created or loaded during the current game session are retained until the game is restarted. This includes tags present in one world and not another, as well as tags that have had their associated file moved/deleted/renamed in the current datapack after a reload.  Possibly related to MC-248621.

How to reproduce

1. Create a new entity_tag file
2. Reload the datapack in game using /reload
3. Delete or rename the tag file
4. Reload the datapack with /reload again
   →  Notice the now-missing tag is still suggested as an option when using the type selector and that the output log mentions that the the tag is not present in the datapack.
5. Exit the current world and open another one
   → Notice that the bug persists even in this separate world