Reproduction steps:
1. Open the user-attached project "Broken Reimport Test.zip"
2. In the Project Settings, under Editor -> Asset Pipeline, Enable Parallel Import and set Worker Counts to 4
3. Restart Unity
4. Under "Assets/Blend" Select multiple .blend files and choose "reimport" (you could also do reimport all, but that takes longer)
5. Check the Scene View, generated meshes are mixed up or missing. Repeat step 3 and get different results each run - non deterministic
6. Disable Parallel Import, set Worker Counts to 1, Restart Unity
7. Select all .blend files and choose "reimport", it imports correctly now
Expected result: Using Parallel import does cause mixed up or missing generated meshes
Actual result: Parallel Import breaks Blender file import causing "Missing Prefab with GUID" and mixing up meshes non deterministically
Reproduces on: 2021.3.6f1, 2022.1.11f1, 2022.2.0b2, 2023.1.0a4
Could not test on 2020.3.37f1 due to project errors when downgrading
Notes:
-Reimporting the meshes one by one does not cause the issue
-User noted that you probably need to have Blender 2.93 (something below 3.0) and Python installed for the .blend import to work.