Is this a bug or not?

It's correct. Yoda gives him Tenacity Up and then provides his buff to the whole team. Buff Immunity blocks it.

^correct. Yoda has to gain the buff before he can spread it.

Ah, cool. Thanks for the info... well not really it is meant to be a bug so I can blame my arena losses on it :'(

Sorry, but I think there is a mess here.
"Example: Yoda has buff block, no other ally does. Use Battle Meditation and no ally gains buff immunity"
Yoda's Battle Meditation grants every other ally just POSITIVE status effect - buff immunity is not positive.
Yoda's Battle Meditation is a little bit different because he should first gain Tenacity up and then give it to allies ,obviously buff immunity blocks it, so there is nothing to give.
I am not sure, but I think that if you use different mass buff ability it should be granted to allies which does not have buff immunity - for example AA Tactical Genius should grant buff to allies which does not have buff immunity

EDIT: anyway this is not a bug

"Aizfgas;862986" wrote:
So buff immunity means unable to get a buff.. Why doesnt it means also to be not allowed to buff something?

I think it does not mean - just Yoda's BM grant ally all buffs which he has - so if Yoda doesn't have any no one have it.

"Aizfgas;862986" wrote:
So buff immunity means unable to get a buff.. Why doesnt it means also to be not allowed to buff something?

Because that's what Ability Block is for.