Is it a mod compatibility issue?

No

If it's a mod compatibility issue, please provide the link(s) to the incompatible mod(s)

No response

Minecraft version

1.20.2

Describe the issue

Some entities, in this case arrows, have a visual bug (Z-fighting) when their reflection is viewed in a mirror.

However, this issue doesn't occur in the reflection of normal portals created with the helper wand.

#1352 describes a similar issue with a mod, and I tested the bug in the latest version to see if it was fixed.
However, I shot some arrows and noticed that they have the same issue, even without the mod installed.
Therefore, this seems to be a problem with some kind of entity rendering inherit to Minecraft itself.

This issue was tested with the Fabric Loader 0.14.23 on Minecraft 1.20.2.
Along with the mods Fabric API 0.90.0+1.20.2 and Ears 1.4.6 (disabled later) on version 4.0.2 of Immersive Portals.
This issue was tested with a fresh instance in Prism Launcher, only setting "pureMirror": true for clarity in the reflection.

Here's a picture that can be compared to the previous example in the post.

Interestingly, the arrow reflection in the upper-middle doesn't display the bug.