Subclass spells stays when changing Class

I’ve noticed that when you settle a class and subclass to a character, if you change the class, the subclass turns empty (it does not turn to “None” or other subclasess). And then, the features are updated to the new class, but the spellbook page stays as before.

I’ve seen this when having a Fighter, Eldritch Knight character, and turning it to a Bard. The spellbook stayed as the Eldritch Knight until I choose “none” or other subclass as subclass