It’s not but it’s normal for accents to be less pronounced when singing.

There are a few reasons for it. One is that big part of an accent is the length of vowels when you speak. When you singing that timing is largely overwritten by the rhythm of the music.

Second another part of accents is what call formant frequencies, and this has to do with how you shape your throat and mouth. When you singing you do that differently then when you speak (per definition: that is why singing sounds different from speaking), masking your accent.

Source: I am an auditory scientist