Spawners' and trial spawners' interior faces are culled when they are adjacent to a full, solid, opaque block (MC-268807); this sometimes happens incorrectly (MC-266463). This does not happen at all for vaults (MC-268242). For any of these blocks, when an interior face is not culled and is adjacent to the face of another block, z-fighting results; this can be easily seen through a spyglass at moderate distances.
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Can confirm this is also present in 1.21 full release

Fixing this issue requires changing the block model, so how can fix this issue and also have the back faces rendered?

Fixing this would be easy actually. Just cull the blockface that's next to another block. This already occurs when a spawner/trail spawner is placed next to a solid block so idk why it's not the case with translucents.
Vaults don't have their blockfaces culled at all so this z-fighting also happens on solid blocks btw

This also happens when spawners border the incomplete blocks such as slab, so I don't think cull the back face of spawners when border these blocks is a solution.

Then the only way to fix it that I can think of is copying leaves block behavior where the inside faces aren't visible at all

Maybe it could be fixed by making the thickness of the sides 0.01 and having both the outer and inner sides be rendered.
Can confirm