Mercy not favouring the weak

What you were expecting to happen, and what actually happened?

My team has 3 members left, in following order: valk , mercy, crimson bat
Valk has 1 hp, no armor, no full mana
Mercy has enough health and armor (filled and ready to cast)
Crimson bat has about 4 armor i think but plenty health

I fire Mercy to boost up my valk, and she targets crimson bat.

What i thought was that she would always target the weakest ally.
I let this pass once, even twice, but it’s now the third time so it does happen more often.

I do not have a screenshot because i cant react faster than her spell, nor do i have a video playing on mobile (ios phone)

Interesting. I’ve used Mercy in hundreds of matches and have never seen this happen.