This might relate to
https://forums.mirc.com/ubbthreads.php/topics/266536/if-operator-regression

For how this affects /set &, which in doubles mod is returning a signed 31-bit number...

Since bigfloat mode has no 2^n against which to be 'signed', does that mean "set %var.bx 4294967295 & 4294967295" would remain as -2147483648 but for %var.bf it would return non-negative results like $and(N1,N2)