Error 500 creating a nested storage location

[sochartgit]

1. Version running is v2.12.0 (latest at the time of writing)

2. I created an item and chose 4200->1 as storage location.
   Storage 4200 is new to the system, creation worked.

3. Add another item and chose 4200->2 as storage location.
   Got the same blue indication about the "New" state

4)Click "Save changes" at the bottom of the page. Bam, error:

5. Open a new tab and go to Tools/Edit/Storage Locations. Item 4200->2 does not exist.

6. So I add it manually (by the way, typing in this window is so sloooooooooooow, and I don't use any Adblock
   or script or anything)

7. Now item exists:

8. Go back to the part window and click "Save changes" again, now another error arguing that
   the element exists, of course.

9. Empty the storage location field and type it again. Now, there are 2 !!!!!!!

10. 1st one is not working, 2nd one is working. Click "Save changes" again, now it works

[sochartgit]

bounce, still the same error. Really annoying.

[d-buchmann]

I tried hard (on 2.12.1 bare metal to be fair), but cannot reproduce this.
Could you share the exact sequence of button clicks and text entries that you did in steps 2 and 3?
Are you intentionally running the experimental FrankenPHP docker container as the logs suggest?