Hello,
I think this is right. There is definitely some bug that doesn't allow addons to receive damage reports. I have tested:
- MobHealth3 - not working at all, tried with different setup,
- MobInfo2 - works only partially, when I also use TinyTip addon I receive information in the frame of TinyTip, not in standard client tooltip frame, neither in Unit Frame,
- MobInfo + ShadowedUnitFrames - not working, I mean it doesn't show mob/target absolute HP, tried different setup and also ShadowedUnitFrames has many options to set to show HP in different styles, also doesnt show target absolute HP in unit frame even if TinyTip is loaded.
I can assure you all 100% that in RETAIL to see mob/target absolute HP it was absolutely enough to use MobInfo or similiar addon. Even in Vanilla!! How can I be so sure about this? Just because I remember!!! Played RETAIL WoW since open beta until the release of WOTLK. Also in few days (when I come back to my home from a business trip) I will try to upload few screenshots from my gameplay.
This situation definitely needs a fix...
EDIT:
As a proof that it was possible to see boss/mob/target HP I would like to paste a link to video of my former guild Muffin Vendor kill of Supremus: https://www.youtube.com/watch?v=3DWM7XZclOg
This video was released in 2008.
EDIT2:
And also to proove you that Muffin Vendor was my guild at that times I found a guildrun movie from Gruul kill. You can see that one of the raid member's name (Group 2) is Slavedriver. Guess what, my char here on Warmane (also Troll but a Warrior this time, not a mage) has the same name. Link to the movie: https://www.youtube.com/watch?v=PCW7JeB68yE
Video released in 2007.

