This is a repeatable issue and has been bugging me for months, I just hadn't written it up yet. Publisher doesn't remove all of a character style's attributes when removing the style or there are some hidden character attributes lurking waiting to cause issues when a style is removed.
Create a paragraph with paragraph and character styles applied to a paragraph - in my test doc, I applied Heading 2 (para) and Date (char)
Press Return at the end of the paragraph - Heading 2 has Next Style set to Subtitle so the new paragraph is set to Subtitle (para) and Date (char). Good.
Change Date (char) to No Style. Now the Text Styles panel shows just Subtitle with no overrides. Good.
Type a character of text - now the Text Styles panel shows Subtitle + font weight overrides, and sometimes I get size, too.
Here's a test doc and a screen recording.
Style Test.afpub
Screen Recording 2023-01-01 at 4.12.01 PM.mov