Expert template settings is not saved

Are you possibly referring to 🐛 Field formatter settings form and AJAX issue (regression on 10.1.x after #857312) Active ?

Thx, Hervé!
But this pitch not worked...

I'll try:
Set up Expert template for field.
Set field class "use-ajax".

But after Save class have'nt value...

Then it might be something else.
Are you sure you enabled a DS layout first? With DS, we first need to enable a DS layout on the view mode before we can use DS field templates. This often comes up.

This is most likely a duplicate of 🐛 Field formatter settings form and AJAX issue (regression on 10.1.x after #857312) Active , so closing tentatively.

If I select "No display" to avoid this error, the "Expert" field formatter settings are not saved with no error message. Just in case anyone comes across this. Patch worked in 10.1 for me.