Around here it seems to be spelled and pronounced bat mitzvah not bas mitzvah. Been to enough of them to be pretty sure about it.
Seems theres some variation.
‘Bas mitzvah’ would generally identify one as from a more traditional Ashkenaz background where Yiddish was likely spoken at least at one time in the past.
‘Bat’ would indicate being more comfortable with modern Hebrew, or perhaps from a Sefardi background.
The same breakout is sometimes evident in pronouncing something as common as Yis-rah-AYL vs. Yis-ROH-ul for Israel.
Don’t ask me why, but the Orthodox Ashenazi pronounce the letter “Tet” with an “S” sound: thus “Bas Mitzvah,” “Shabbos” and “Parashas” which should be “Bat Mitzvah,” “Shabbat” and “Parashat.”