'-x to maximum energy shield' does not work

'-x to maximum energy shield' modifier on a ring does not affact the maximum energy shield

Your helmet has "increases and reductions to energy shield instead apply to ward", so the ring won't change your energy shield.

Added/subtracted flat values are not increases or reductions.

This was also an issue during Kalandra league, but I don't think the functionality ever changed. It doesn't seem like you can have less than 0 ES from both your rings combined; I'm unsure if the issue also extended to amulets.