I may have encountered another bug concerning the Necromancy of Thay:
Normally, no character tags were visible for me under the tag category in the character sheets.
But after my character had read the book, character tags for all companions and Tav were visible, including a tag
for my Tav.
When I gave the book to Astarion, the character tags remained invisible, until his cinematic dialogue, where he tries to read the book or rather argues with it.
Astarion did not get the aforementioned tag, neither did my Tav in this case.
The character tags remain visible until saving.
They are still visible directly after saving, but after loading this save, they are invisible again.
I hope this description helps to narrow down the search for the cause of this bug.
And thank you for your hard work! I am very much looking forward to August 31 (and I will keep on playing in the meantime, I hope I can finish some playthroughs with different classes before the full release).
