UnitExists("Player-Realm") false when he's standing here

So for years now the Unit* functions have accepted player names as well as UnitID strings. I can’t find anything that suggests this might have changed in 9.0.1.

I’m trying to debug Raeli’s Spell Announcer, which is a helpful thing to print stuff to frames when spells start/stop doing stuff. More specifically, I’m trying to gather enough information to submit a useful bug report to the author. It has an option to whisper people for combat buffs like, say, Power Infusion. Its logic is

local full_name = value extracted from CombatLogGetCurrentEventInfo()
if   <supposed to whisper> and  UnitExists(full_name)  and  <it's a whisperable unit>  then
    <send whisper to full_name>
end

The problem, however, is that UnitExists is returning false. I’ve tested it on friendly units both on my realm and cross realm, and player names that aren’t my own all return false.

What am I forgetting here?

Huh. Going blind. Cheers!