Color label not written to sidecars

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

I just labeled 700+ files with star ratings and color labels.
Now I find DT did not write the color label to a single file.
I don't know how to reproduce

Steps to reproduce

Go to light table
Color label an image

XMP file has a line "xmp:Label="NoColor" " Which is incorrect because it does have a color.

Close and re-open DT.
The image has the correct color label but it's not in the sidecar file.
The rating is in the sidecar.

Expected behavior

Write the correct color label to the sidecar

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

5.2.0

What OS are you using?

Linux

What is the version of your OS?

Mint

Describe your system

No response

Are you using OpenCL GPU in darktable?

Yes

If yes, what is the GPU card and driver?

not relevant but it's the intel CPU

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

Start by reproducing the report in darktable 5.2.0 on Linux Mint: color-label an image in the light table, inspect its XMP sidecar, then close and reopen darktable. Done means the sidecar contains the correct color label instead of xmp:Label="NoColor", while the label remains correct after reopening.