Iirc, the word “prove” in this context is the archaic definition “test” e.g. the proving ground. This would imply the original meaning of the phrase is in fact the opposite of how it is normally used today: "the exception proves the rule "means ‘an exception tests [whether or not it is] a rule.’ As you say, people now use it in this strange fashion where the existence of counter evidence somehow proves the point
Iirc, the word “prove” in this context is the archaic definition “test” e.g. the proving ground. This would imply the original meaning of the phrase is in fact the opposite of how it is normally used today: "the exception proves the rule "means ‘an exception tests [whether or not it is] a rule.’ As you say, people now use it in this strange fashion where the existence of counter evidence somehow proves the point