I think I figured out the root cause.
Floor joists are 2x10 (actual 2x10, not the 1.5x9.25 you get now).
The joist in front of the toilet is notched to about a 2x3 for the down pipe. Then there's a sister joist of 2x6.
So it's weaker than it should be. So eventually it sags, then leaks, then wet and rot makes it weaker, more sag.........
BTW, house was built in 40s.....
If you haven't already, add a support at the weakest point. If there's enough room a pier block and strut, if not dig a small, sqaure hole with a flat level bottom at least six inches deep and fill it with concrete and strut from there. Just my $20 (the new 2c)
