From the announcement:
the badge counts distinct tag wikis you edit excluding excerpts
and the FAQ:
Edits to tag wiki excerpts do not count [emphasis mine; thanks @bobble]
Update: I've now updated both links above with the answer explaining the "oddity".
Before, creating/editing tag excerpts never counted towards the Research Assistant badge as shown above, now recently they started to, while the description hasn't changed:
Edit 50 tag wikis.
Is it an unannounced change that needs reflection in the description?
Or a bug?
Or was a bug and is now fixed (excerpts suddenly being part of wikis)?
Also semantically, a book excerpt is not a book.
I noticed the change very recently, because I was 48/50 and that badge's progress is always on my profile, and two excerpt edits later (latest), I just got the badge (not here).
Repro #1: https://chat.meta.stackexchange.com/transcript/message/9014885#9014885 (thanks @MetaAndrewT.)
PS there were no pending wiki edits; I've had the editing privileges for years, in fact I made only one suggestion ever, and it wasn't tag related.