Have been trying to get this challenge done for a while. I would get a ghast through a portal but he despawns instantly on the other side. Anyone know what is going on?
Maybe the portal is too small and the ghast suffocated.
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If you're on a server or you have anything that keeps chunks loaded while you are away from the Overworld it could be despawning due to being in a loaded chunk with no player nearby.
Ghasts can be spawned with spawn eggs in the Overworld so there's no change that makes it impossible to have ghasts in the Overworld in 1.14.4
I just got the advancement in creative mode 1.14.4 by spawning a ghast in a 10 X 10 X 10 or so box with all 4 walls being portals.
The Overworld portal was the standard 2 X 3 and that caused no problem, the ghast seemed to be stuck between the portal and a tree but wasn't taking any damage.
(I did have to use all 4 box portals myself first, otherwise the ghast just passed through the portal remaining in the Nether.)
Have you tried nametagging the ghast?