How do you know? Is it leaking downstairs? How do you know it is
leaking AT the drain flange?

Water was leaking from the tub and coming out from under walls in adjoining
rooms.

I did a test where I removed the drain strainer thing and sealed off the
normal opening (but not beyond) with aluminum tape. I ran about a gallon
into the tub and watched the level drop. I think it was gone in just over
a minute. Surprisingly, given the very small water level, I actually saw a
very tiny vortex (whirlpool) develop over the left edge of the drain
flange!