The Meaning of Life, the Universe, and Everything.
I tested pushing ghasts (from nether to overworld) through in a creative superflat world, once the input portal is large enough, it works as a charmed, size of destination portal may block the ghast or it can end directly in the sky, but it is there in the overworld.
I try pushing 4 or 5 ghasts from and to big portals on a multiplayer server in survival mode, even tried nametagging the ghast, no ghast in the overworld.
The only big difference I see between both situations is that on the multiplayer server in survival mode, the overworld portal is located high above an ocean, while in the superflat world, it's on the ground in a plain.
Any clue what prevents me to bring ghasts in overworld on the multiplayer server?
how are you pushing them? If you've got them in minecarts, that might be an issue (I seem to recall it being said mobs-in-carts can't go through portals.)
Another thing to consider is that for mobs there's a cooldown timer of about 60 seconds before another mob can be sent through.
Finally, with the small hitboxes that ghasts have, perhaps if it's trying to go somewhere it's simply not staying in the portal long enough to trigger teleportation (ie, the hitbox is outside of the portal block, so the game thinks there's nothing using the portal)?
I can actually say I've also had this problem on a multiplayer server & no matter how I tried (pushing through/minecarts/fidhing rod etc) the Ghast disappeared from Nether but never appeared in overworld....
.....UNTIL I used an ALT account in the overworld by the portal & used my main account to push the Ghast through - THEN the ghast appeared in the overworld
I'm going to guess that on the Multiplayer server the 'overworld chunks' were not loaded for some reason & so when Ghast went through the game didn't know where to spawn it in the overworld causing it to 'despawn' - WHY it works in Single player though I don't know