I think I have found a bug in the effective stack filter.
Checking a huge database had negative winrate in the short section, something that is totally impossible
Investigating I have managed to obtain the problem, In HU games all the hands in which there is a walk the filter considers that the effective stack is 0.
Image attached, stack effective filter = 0 and the walks appear
https://gyazo.com/468204909c26972c391d5ce72b6930aa
effective stack filter between 1 and 11 and negative winrate (when the walks are not present)
https://gyazo.com/642a8c0dd736436be1c08d7dd7928a33
all player filter with effective stack 1 and 11 ( the winrate should be 0)
https://gyazo.com/3cba05632a1b7aaa352ded1aa6a19781