In versions across 1.20 to 1.21.1, when using Bliss shaders with the Ultimate car and Ultimate plane mod, the entities will be rendered transparent. Tested on both Forge and NeoForge oculus/iris versions.

Affected mods:
https://www.curseforge.com/minecraft/mc-mods/ultimate-car-mod
https://www.curseforge.com/minecraft/mc-mods/ultimate-plane-mod

Logs (no error recorded)
https://paste.atlauncher.com/view/4a176f30-4c3a-4ffc-86bb-dd9ca2655b0b

Images:

try disabling this settings: shader settings -> debug/experimental -> TRANSLUCENT_ENTITIES

Can confirm having this issue as well on my end.