I'm started to use @globalista-dev's fork of IronChests, and when i opened a cooper chest (that have more slots than regular chests, of course) the sort button become off-position:

(ignore the different icon, i'm using the quark button texture via resource pack).

I dont know if this is an issue on your or globalista side, but by heuristic i opened the ticket here first.

I might be wrong, but I recall using the original Iron Chests (before my fork) with Inventory Sorter and had the same problem.