I noticed that the projectiles of the gatekeepers get stuck in palmtrees for some strange reason,

This is probably a ZDoom bug. All I can think of is that the combo of RIPPER + HEXENBOUNCE is causing something to botch.

Why has this been tagged as fixed? Is it really fixed, or just closed as a ZDoom issue with no action taken?

Closed as ZDoom issue

Reopening that one because it still happens and it's unlikely to be addressed ZDoom-side.

So, how to fix this on our side? Here are a couple possible easy solutions:

• [wiki=Actor_properties#BounceCount]BounceCount[/wiki]. Give it 12 bounces or something, should be plenty enough.
• A_Countdown can break a loop in which it is inserted

It does have BounceCount set. For whatever reason, getting stuck in the tree will refuse to decrement the count despite clearly attempting to bounce repeatedly.

An A_CountDown loop could be introduced to prevent it from sticking forever, but it still wouldn't prevent it from occurring in the first place, which is hella weird.

Maybe a velocity check would do?

Aha! Figured out what the problem was. The stuck projectiles were caused by AoD pipebombs impacting trees. I noticed the bombs were still using A_CustomMissile to spawn the sub-munitions, so a quick switch to A_SpawnItemEx did the trick.