I’m relatively good with tongue-twisters too, but I still mix up sounds which I didn’t grow up using regularly if I’m not very careful, e.g. sometimes when speaking English quickly I accidentally use the Italian-style r (which sounds more or less like the English flapped t) instead of the English-style one, leading to misunderstandings like “what is” when I mean “where is” and the like.
It so happens to be that I can do most tongue twisters with little effort, and that normal sentences are rarely tongue twisters.
I’m relatively good with tongue-twisters too, but I still mix up sounds which I didn’t grow up using regularly if I’m not very careful, e.g. sometimes when speaking English quickly I accidentally use the Italian-style r (which sounds more or less like the English flapped t) instead of the English-style one, leading to misunderstandings like “what is” when I mean “where is” and the like.