arena3 is a hunter, this feels like on Blizzard's end, since 3 is showing up, if my addon hid the health bar, it would've hidden the number as well.

Unlikely to be related to cross faction, the hunter is night elf.

Between solo shuffle rounds, nameplates might be reused?

Can see the name on top of nameplate, and can even see debuffs.
Both are a part of the health bar, so this might just be a blizzard bug on hunter feign death.

Seems like a Blizzard issue, other folks are complaining about it too.