Ran an 8 Fungal Folly with Alliance toons. Was unable to heal them barring notable exceptions like Holy Prism.
Edit: In addition, after leaving a cross-faction delve, player names from those on my faction turn orange, as if they’re in another faction. Fascinating stuff!
Same, I wasn’t able to directly heal or res the alliance member of the party but aoe still splash healed them. When the delve finished and people started leaving, the alliance player was the last player besides me to leave. Once they left, Brann became “mine” and was attackable. I assume this was because he was most recently alliance.
LMAO. Amazing that it released in this state.
I had similar issue this morning except the other person in the party was not a different faction, we were both horde, but she turned orange and I was unable to interact. Brann similarly turned orange and I was unable to interact with him as well even though I was the leader of the party. Def made the Delve harder.
Also happens if one is pvp mode and one isn’t
Same, this happened to me today. Ran three delves all no issues, then go into this one and cant target to heal anyone and all players are now unhealable even in Dornagal.
I’ve been having the same issue on and off.
sometimes rebooting the game fixes it, but it seems like a temporary fix to the issue and it comes back later.
thankfully it seems that i can still aoe heal opposite faction, but thats hardly a very effective way specially if its the tank i can’t heal properly lol
Ran a bunch of delves yesterday and today with no issues but now i can’t because everyone becomes unhealable upon joining the delve.
Great quality control Blizzard, as usual 
Had major issue with cross faction delv, could NOT single target heal. Only my AoE type heals worked. Other players of different faction name bars were orange. I even disabled ALL addons and re logged. No change. Still couldn’t heal my party.
By the way, this bug can also happen with same faction, although it seems to happen overwhelmingly with cross faction. I have had it happen with same faction on at least a few occasions myself (they may have been in warmode or something; I don’t remember).
Just have the person zone out and zone back in and that fixes it. If everyone is hostile, the problem is you - so you should zone in and out.
Addons/reloadui/rebooting, etc won’t fix in my experience but a simple rezoning works every time, although sometimes its you who needs to do it and sometimes its them.