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My suggestion is to try the latest devbuild at https://devbuilds.drdteam.org/gzdoom/ . Sometimes the problems are fixed without closing this one.

Another suggestion is always to fill bugs with all the details.
For example: with a 16:10 resolution the texts are correct, with a 16:9 not.

About my previous post: I was wrong. The Morph Ovum in Hexen is present but not working even in vanilla!

I hope these bugs aren't going to be left unfixed for too long, they're gonna be baffling or even off-putting for first time players

Yup, I can confirm even with the GIT version.
If needed I can open an issue to Github repo.

EDIT: with the latest unstable version g4.13pre-66-gae28eeae9 looks like in Heretic the morph ovum works correctly, and in Hexen the porkalator works correctly, too, but not with the morph ovum.
So I've created this issue with all the details I've found : https://github.com/ZDoom/gzdoom/issues/2583

Yes, morphing appears to be bugged in 4.12.1. It's not just Heretic and Hexen, though other games don't have morphing without mods. It seems to spawn a morphed monster while a separate monster of an original class still exists (but invisible and invulnerable now).

I found couple of problems that both Heretic and Hexen share. One is the story text not fitting the screen for pretty much all text screens.
And the morphing enemies with the Morph Ovum and Porkulator turns them invisible, yet still morphs them into chickens or pigs depending on the game.
I'd post screenshots but it looks like this forum doesn't let me. I hope these bugs get sorted out though.