That's the thing though, 忘れちゃ is a completely different, unrelated form that means 忘れて+は. So yes, this split is in fact incorrect and if it wasn't past tense, b

That's the thing though, 忘れちゃ is a completely different, unrelated form that means 忘れて+は. So yes, this split is in fact incorrect and if it wasn't past tense, but dictionary form 忘れちゃう, the result would be even worse with う being split off as "rain"? or something.