Slither being auto-unlocked as Vyper's first ability is not accounted for in the ability point order, so each upgrade is off by one (i.e. selecting 1st upgrade is listed as unlocking it it, 2nd upgrade as 1st, max upgrade as 2nd). Slither can't be selected again so max upgrade can't be added to the ability point order. I see some public builds with Slither maxed and some that don't so I don't know if there's a workaround, but I know the bug happens if you make a build while in Hero Testing.
