[Merulina Guardian does not give it's buffs if Merulina is at full health.]

Aaaand this is why you should use Combat Discipline while using Yareli, the Augment will ONLY proc if Merulina is healed by the first ability. 

[Deadly bug (spoilers)]

So, this only happens if you're not the host:

Changing back to Warframe While using the Operator or Drifter will kick you out of mission while making you invisible and when mision is completed  you get no rewards/will get you stuck facing a single position and can't do anything and you need to die to undo that,  this has been a thing as far as i can remember, and needs correction as fast as possible.

Btw Yareli top tier frame