Are you really sure you have it Resistance NOT present? It's the sort of thing that's so obvious its easy to overlook.
Yes, 100% sure. And both west and east starts within their respective trigger areas (the triggers are there to determine when the mission should end). And triggers are very, very global AFAIK.
I made a workaround, using a count this trigger. But I would still want to know the answer to the original question.
EDIT: Ok, hereĆ's the solution...
The condition of the NOT PRESENT trigger must be set to:
local server AND this (instead of just "this")
This should have been quite obvious really, because a not present trigger is actually a trigger that counts units. And all counting should be done only on the server (according to Snypir's MP guide). For some reason I overlooked this. I'm now considering this topic solved. Thanks for all the input.