As others note, the first FoF blast hits like a truck, and if you aren't at full health you will die. I've got my resource usage planned out such that I go into that with two addersgall charges -- one to heal myself up after that first blast that I know I'll need, and one as an in-case if I need to heal myself up before it -- so usually am safe there, but if we've had stumbles earlier in the fight and either I or my co-healer is low on juice due to unexpected rezzes and healing-up-the-revived-folks, there's a very high likelihood one of us will die to that first blast even if we're almost at full.

That said, there's certainly the possibility that the snapshot is being ridiculous and not letting a heal go through before the damage. My static's main tank is in Paraguay, and his ping to our data center is... let's just politely call it "not great". There were times last tier in E12S (when he was DPS) where I or my co-healer would be standing literally on top of him, cast an AoE heal, and he'd still not get healed by it when everyone else in the party -- all at greater distance from the healer than he was -- did not. With the result that he -- and only he -- died to the following raidwide. You wouldn't think the player ping would affect server-side things like that -- and thankfully, we haven't had that problem *this* tier -- but so far as I can tell this game's netcode is vaguely cursed (and most likely haunted).

¯\_(ツ)_/¯