Tried something different, a bleed build with upheaval and full set of Sescheron’s Fury charms. Now the damage number for bleed in upheaval tooltip says -2,147,483,648. Only in combat areas, in town the number looks normal.
I have experienced the same with a bleed build using flay.
It looks like the damage value for this skill is stored in a signed 32‑bit integer. When the calculation exceeds the maximum storable value (2,147,483,647, which is 231−1), it overflows and wraps into the negative range. That’s why the damage stops scaling and shows a negative number.
I’m not sure whether only DoT effects use 32‑bit integers but this is definitely a bug.