Tested with 305, 304 and 306 version neither of them work.
Checked raids from: legion, shadowlands, BC and neither of them are showing the tooltip of items or achievements. here are some screenshots

Hovering the boss to show proove it has things to be checked:

on minimap:

Shadowlands:

BC:

Draenor;

I can't see any way to use the new tooltip API functionality to hook into these tooltips. I'm pretty sure Blizzard still has many bugs to fix with the new tooltips since there are many other tooltips which don't register their contained content when being rendered either. We will have to see if Blizzard goes about fixing them, or if they specifically want certain tooltips to be unable to be modified by addons.

Made a temporary fix to this in #1355 until blizzard fixes the tooltip API