Don't know why this is happening yet, just pretty sure that it is.
📌
Slowly, very slowly start OOPifying the render system
Needs review
converted StringTextfieldWidget to use the new API.
https://git.drupalcode.org/project/lms/-/jobs/6078870 is the LMS → module's next minor test failing.
When you revert the conversion, the test passes.
I'm hoping this is due to a silly mistake in the conversion that core test coverage didn't catch, and not a symptom of a broader issue.
Active
11.0 🔥
render system
Crediting @graber who actually found the bug and bisected it - we were discussing in slack.
So far found out that for some reason on AJAX call the resulting textfield element '#value' is set to an empty string after the change which prevents FormBuilder::handleInputElement() to set its value from user input.