Master cylinder failure. Buy a LH or RH sram hy'rd kit from anywhere that has one, they are less than £20 rrp, you don't need the metal plate thing, only the piston top and spring, o rings etc in the kit.
Fairly simple to replace, follow the numerous youtube videos, think SRAM have a proper guide as well.
Happens to a lot of them. Perfect bleed, gentle braking and it kinda works OK, but as you pull master harder it just goes to the bars, no leaks, no loose stuff, no seized pistons etc. Almost 100% this will be your issue.
I'm a shop and I charge £20-25 labour to fix bleed and test ride.
YOu cannot buy it from sram, its not an ISO size. Do not loose it Dab it in grease and work in a clean area.
I've lost one before, utter ball ache.