Apparently disabling sky fixes these issues.
https://mclo.gs/abvmn7h for mod list

still have to gather more information

I don't seem to be able to reproduce this, the skybox blending usually looks smooth. If anyone has this issue please respond to this. Turning off the sky is a temporary fix.

what sky are you using though?

I'm currently using FAT's Sky with

• FabricSkyboxes latest version (v0.6.1+mc1.19.2)
• FabricSkyBoxes Interop latest version (v1.1.3+mc1.19.2-build.25)
• Nicer Skyies latest version (v0.1.0)
• MCPPPP+ latest version (v0.7.3)

The sky is not converted using MCPPPP+ application, rather converted using Interop
My modlist: latest.log

Not having any skybox issue like your given picture

Apparently no custom skybox was being used, the person had just customised something about the vanilla sky that caused the smooth transition between the different parts of it to disappear. If anything similar ever happens again I'll look into it. Else, I'll close this issue.

here are examples

Will close as it hasn't manifested itself ever since.