This isn't a new bug. It has been like this since they "fixed" taunt back in 2016. Since then a successful taunt, unless it crits, will not snap aggro. Mobs that have been taunted successfully, but are still attacking someone else have returned the "you already have your target's attention" message since that patch. Since that patch, I have had to successfully taunt AND do something to the mob (darkness, terror) to snap aggro. That patch has made for a lot of really bad tanks that had no idea that the mechanic had changed over the last 4 years.
You read the first paragraph then pressed reply. Read the rest of the post. The non-aggro taunt is the setup scenario for the bug.
My bad. I missed that part. This has been a thing since 2016. Obviously, working as intended. Can't comment on this part as I've never tried to taunt and then buff.
