For my impossible-project, I have -a lot- of units set for the Player 15 (Hostile player).
In my latest test, I've seen that some units were removed. Precisely, it were 3 Banelings, a bunch of Zerglings and an Immortal.

Our map contains a total of 1384 units.

Could it be I have reached the maximum of units a player can control? Or is it possibly a trigger messing with me?

Rather more a trigger.
There is no reason why blizzard would add a unit limit PER PLAYER and I don't think that the unit limit of sc2 is below the unit limit of sc1.
I'll make a test map, hold on.

EDIT: Created 2592 hellions for player 1 in the editor, sent out a text message of the unit count for hellions, displayed 2592, so those units were not removed.

Thanks!