I will often receive plant layouts (in 2D DWG) and be asked to put equipment that fits the layout. The image above is an mocked layout (made from two sketches: one top view and one side view). It's a SLDPRT file.

I will then create an assembly; insert the layout SLDPRT and whatever components I need; make the necessary mates (e.g., a component front plane coincident mate to a point/line in the layout SLDPRT); and then save and close. Similar to the image above.

I will then open another unrelated project and work it for a while - saving and closing after I'm done.

Later on, I open my old assembly because i need to make some changes and this what I see:

If I check the mates... sure enough Solidworks has mistakenly picked the wrong sketch point.

Why does it do that? Nobody but me has touched the assembly.

Also, this behavior is recent and random... I've been doing this for MONTHS with no problem. It just now randomly do this - out of the blue. Very confusing and frustrating.

P.S: the images above are mocked. The actual assembly and sketch SLDPRT are more complex.