This is a regression apparently introduced by MDL-67968

Some change into form structure/classes (fieldsets) have caused certain elements to start displaying incorrectly.

See for example https://github.com/h5p/h5p-moodle-plugin/issues/329 where the problem has been detected.

At the time of writing this it not clear which other stuff may be broken in a similar way.

Quoting tusefomal:

I see that this element fieldset goes from flex to a regular display with this change, adding a "w-100" class will increase it's compatibility with previous forms

Ciao