Ok, I checked this Solid with open surface Script - it's clean. I did Edit -> Join. And selected only one edge absolutely clean edge between two other edges - and it still creates separate geo not doing fillet at all. Ok, obviously there are some unseen errors in geo - is there a way to check for those other than just Edit-> Join?

Since you have the model can I pls ask you to try filleting one edge of this solid? You will exp. it yourself and will have a clearer picture. Thanks!