When I started on the 1911 in my teens, my father drilled into my head that I was not to draw with the thumb on top... Always under.
It just became 'old hat' to present the firearm in that way, that is, thumb under the safety until ready to engage.
He was lefty, and taught me on an old Remington Rand that had an ambi.
I'm lefty as well, so employing the m&p2.0 was the same process.
However it was near impossible to get my thumb to work with the Sig 938 when I owned one.
Like to think that like anything else, it comes down to conditioning.