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Not at all.

[url=https://github.com/coelckers/gzdoom/commit/d9db8984af603b1d75f03b182adf1ebc55334f14]pr merged[/url] thanks

It was not a software renderer bug, it was the same as the one in the legacy branch.
I've made a PR: https://github.com/coelckers/gzdoom/pull/1400

Class SkyChangingZombieman : Zombieman
{
    Override Void PostBeginPlay()
    {
        Super.PostBeginPlay();
        Level.ChangeSky(TexMan.CheckForTexture("<FWATER1>",TexMan.Type_Any), TexMan.CheckForTexture("FWATER1",TexMan.Type_Any));
    }
}

I tested this both on GZDoom 4.7 latest commit + GZDoom 4.5pre with Vulkan backends. It is a doom software renderer only bug tho.

Spawning this fella will cause the game to crash when any part of sky is visible.

Supposedly any kind of non-existing texture will do.
[code]
Class SkyChangingZombieman : Zombieman
{
Override Void PostBeginPlay()
{
Super.PostBeginPlay();
Level.ChangeSky(TexMan.CheckForTexture("<FWATER1>",TexMan.Type_Any), TexMan.CheckForTexture("FWATER1",TexMan.Type_Any));
}
}
[/code]
Credit for code and discovery goes to: inkoalawetrust