Pretty serious issue. Mission EH executes every frame and is by the look of it designed to draw ui elements on screen. Therefore it stops as soon as main screen loses focus. Pretty logical, why waste resources if you cant see the result. It also not available on dedicated, probably for the same reason.

However when user leaves MP game and drops to the end game screen or lobby, as long as Arma 3 window has focus EH continues to execute every frame. I strongly doubt this is also design decision.

I have logged client state, frame and game time to demonstrate the problem:

getClientState diag_frameno time

..........
BRIEFING READ 18464 1143.23 <- in game
BRIEFING READ 18465 1143.24
BRIEFING READ 18466 1143.25
BRIEFING READ 18467 1143.26
GAME FINISHED 18468 1143.26 <- game finished screen
GAME FINISHED 18469 1143.26
GAME FINISHED 18470 1143.26
..........
GAME FINISHED 18590 1143.26
GAME FINISHED 18591 1143.26
GAME FINISHED 18592 1143.26
GAME FINISHED 18593 1143.26
MISSION ASKED 18594 1143.26 <- lobby
MISSION ASKED 18595 1143.26
MISSION ASKED 18596 1143.26
MISSION ASKED 18597 1143.26
..........
MISSION ASKED 20568 1143.26
MISSION ASKED 20569 1143.26
ROLE ASSIGNED 20570 1143.26 <- joining back
ROLE ASSIGNED 20571 1143.26

//<- in game, execution finally stops