Cutup’s accent drops in and out throughout Clone Cadets. There's a handful of examples, but for instance, he points out a droid to Fives in the average clone accent, then says no problem with his normal one right after.
It could just be DBB forgetting that he was doing an accent at times. But, I think it could also be that since it isn’t his natural accent, it drops when he’s more stressed or emotional. Kind of like how some people’s accents get stronger in such situations, except he reverts back to the standard clone one.
Consequently, he must’ve tripped up some people twice: once when they first hear his accent and then once again when he reverts back the first time unconsciously. (Can you imagine their reactions? Lol. The real question is how much he notices their reactions.)
On a similar note, he can also consciously drop it when the situation calls for it. He drops it during Clone Cadets when talking one-on-one with Bric, going back to the clone accent. Definitely as a sort of defense-mechanism here, especially since Bric literally beats him up. But the point still stands.
Once off Kamino, fewer situations call for it, but it’s definitely possible. More often than not, though, his different accent comes in handy more than having him switch to the clone accent. Need someone to go undercover with a helmet on or something? Cutup doesn't sound like his brothers, he's the perfect pick. It helps that he's slightly better at lying than most of the rest of the Legion. Yes I'm ignoring canon again and saying he joins the 501st with the rest of Domino what about it