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Fixed in 2019.4.X, 2020.3.X, 2021.1.X
Prefab script field reference is lost when project is upgraded
1. Open the project ("1328724Repro.zip") in not reproducible version
2. Open "backgroundEnvironment" prefab
3. Select "airplane_pivot" Game object and open the Inspector window
4. Inspect Airplane GO script field
5. Close the project and open it in a reproducible version
6. Repeat steps 2-4
Expected result: Airplane GO remained the same
Actual result: Airplane GO script field reference is lost
Reproducible with: 2019.4.14f1, 2019.4.24f1, 2020.3.4f1, 2021.1.3f1, 2021.2.0a13
Not reproducible with: 2019.4.13f1
Could not test with: 2018.4.33f1 (script errors)
Jul 12, 2021 17:50
Disregard my previous comment.
Alternate workaround does not work for whole prefabs. Some prefabs still had missing links.
Use the prefab reimport workaround!
Jul 12, 2021 17:41
Found another workaround (at least, worked for me, after migrating from 2019.4.22 to 2019.4.28)
* Opened the project on 2019.4.28 and found the missing links
* Downgraded to 2019.4.22 (just opened on that version)
* Checked that links were still there
* Reopened on 2019.4.28
* Links were fixed
Jun 20, 2021 04:00
This occurred for me on some prefabs upgrading from 2019.4.15 to 2019.4.26. Thanks to GCCOOPER comment to reimport all prefabs it fixed it. Thanks to archive patch notes I was able to find this bug.
Jun 02, 2021 13:21
Just lost two hours upgrading my project until I found the workaround by
(GCCooper May 19, 2021 22:26). Can't you just automate this process in the upgrade scripts?
May 26, 2021 18:12
Also affected by this issue. Can't upgrade with it breaking all my prefabs, and I also haven't done the "downgrade thing" purported to cause the issue.
May 19, 2021 22:26
Still getting this on 2020.3.8f1 "LTS", which implies long-term support. Best workaround I have found is searching the project window for all prefabs (via the type search button), select all, right click, hit Reimport. Note, Reimport All does _not_ work!
May 10, 2021 09:34
4 out of 4 projects are having issues when upgrading from 2019.4.21f1 to 2019.4.24f1+.
None has anything to do with 2019.3 or whatever downgrade.
The only (partially) workaround so far is deleting the entire Library folder and letting Unity to rebuild it again.
This "Won't fix" is simply unbelievable, on a bug impacting so many projects since several weeks.
May 07, 2021 01:44
May 03, 2021 11:18
Reproducible with 2021.1.5f1 too. Would love to see this fixed.
May 02, 2021 21:21
This essentially make upgrading impossible for any project. Should be highest priority.
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Resolution Note (2021.2.X):
The supplied bug has an asset which we believe has been upgraded from 2019.3 to 2019.4, before being re-opened in 2019.3. At Unity, we don’t officially support the downgrading of assets between versions (though it may work on occasion).
If you have this issue without bringing assets from later Unity versions, I would suggest filing a bug with your project. We’d be happy to look into it!