Apparently :
- as of patch 9.0.5 the friendly requirement given by Archmage Khadgar was no longer in effect and one was able to take the quest and turn it back immediately
- as of patch 9.1 at least from 2021/9/29 (see recent comments here), the requirement is still in effect.
Is this a bug ? I would say so. (I don’t see anything regarding this in the 9.1 patch notes.)
This in infuriating, least to say, especially after having taken the whole Suramar, Nightfallen but not forgotten, Good Suramaritan and Insurrrection (up to An Elven Problem, where I need to do 10WQs in Suramar, hence I need to activate WSs in Suramar, hence I need to complete the Uniting the Isles quest) story lines in 3 days in a rage mode. 
What’s the issue with my bug report for you to point me toward this ?
The issue is that bug reports cannot be made via the forums. They have to be made via the bug reporting tool in game.
Bug reports on the forums just get disregarded and not forwarded to the developers.
It shows you how you report bugs in WoW. Customer Support don’t deal with bugs nor do they pass on bug reports.
You can report them in game and that FAQ explains how 
Ok, I tried to report through the game, but did not succeed. Will try again.
Edit :
Between
- waiting for Blizzard to acknowledge the bug + fix it
- gaining myself the friendly rep level where needed
I took the second option – took me 3h though. Thx Blizzard.
Just to finish on this : after completing the Archmage Khadgar’s Uniting the Isles I naturally went back to the Stabilizing Suramar quest consisting in doing 10WQs in Suramar. I was only given 7WQs, counting 1 pet fight WQ. I don’t do pets, so it’s 6, and tomorrow I’ll have only 2 more (the pet one will reset) so that I’ll have to wait 2 days to finish the Stabilizing Suramar quest. It is indeed infuriating.