This behavior is responsible for messing up so many build queues. If you box select and end ur box select corner at a mex, it will cancel whatever queue you were doing and try to build the mex with whatever unit you've selected. feels super buggy because left click should basically only mean select for everything. Binding anything else to it without you having explictly pressed a hotkey just messes everything up.
I guess a workaround would be if you change it from left click release to left click down or disable functionality while doing box select.