Just doesn’t work and it is very frustrating. They changed it why I don’t know. Is anybody else having issues.
does that other piece of geometry need a shell on it, or some bit of thickness?
Usual story
Use ProBoolean instead. Sometimes the one works, sometimes the other
I ended up slicing it, but why did the change the tool that worked perfectly?
They changed it because it DIDN’T work perfectly. Boolean is one of those things that never worked in all cases since early Max already. 2017 was suppose to fix the issues, guess they just swopped it
The old booleans are still there… I’ll try to remember how to bring it back from the dead.
It has always been buggy, but why change everything, except being buggy? I can see changing it to rid it of bugs.
The good thing they realise is that they have to change to non-destructive workflow. But of course , they are doing it in Autodesk Style 
booleans nnever worked well with “open geometry”..i mean it works well if you have a solid object…and it doesnt when you have something like a plane…at least for me
in your case …i woudl not even bother booleaning it and went directly to slicing and cutting it up to save me from pulling hair…those who know me already know there is not too many left on my head :)))
you could try shapemerge instead. you can use planes as the body and use closed splines (a rectangle). It works really well.
i do that for roads and stuff. Never thought about it for window openings.