I'd have to look up the linguistical explanation but that seems to be the case.
I've never heard singers say it's intentional, but it seems most accents are lost when singing in English. I think it's only certain phonemes that carry over, but most don't seem to be the ones that carry an accent (Jamaican seems to be an outlier). Think of all the British invasion bands. Sabbath really comes to mind. Can barely understand Ozzy when speaking. Or think of Shakira.
Rapping on the other hand is much closer to a speaking voice. While you can intentionally hide your accent, you don't have to as much. More comes through, at least when you aren't actively hitting pitches.
It may be how people are traditionally taught to sing, I don't know. But anecdotally this seems to be the case from the music I've heard.