Likely something for @neil related to the recent migration of #tagging into core. Noticed recently over at Stonehearth that not all tag changes appear in the post edit history. Here is one example:
Tags are restricted to TL4 only, so the user didn’t attach any tags on post creation. I am pretty sure that 8bitcrab added a tag on March 30, followed by Albert on March 31, but Albert’s tag doesn’t appear.
I can provide more examples if needed, hopefully this is enough.