Let's make the situation.

Player A has stack 200, player B 10000, player C 80000, C has Button. A is BB(100), B is SB(50). 

C can all-in 10000 in this system. then C bet 10000.

B fold. A call 100.

then Mainpot is 200+200+50=450 and Sidepot(Remain) is 9800.

if A win, normally A get 450. but in this world A get 10250 and C's stack reduce 70000.

Why?  Maybe in Head's up all-in don't make sidepot in this system.

In Heads-Up mode, no side pots are created; that’s intentional. But you are correct in saying that C’s bet should be reduced to A’s– that’s a known issue. It could be a non-trivial issue due to the limitations of Udon V1 but I would like to fix it. I’ll note it in the bug list when I have the chance.