This is new, intended behavior. Please see https://bugzilla.mozilla.org/show_bug.cgi?id=1907452

It may be intended, but it is bad and a regression. It seriously impairs the usefulness of Calendar.

This should be fixed in 128.2.1.

I can't test 128.2.1 because I am already on 128.2.3 and there doesn't seem to be a way to downgrade. (See https://support.mozilla.org/en-US/questions/1457943) In 128.2.3, calendar search doesn't work at all. The result list always shows all events from the selected interval, no matter what I type into the search field.