I believe that there is a selection mask that will allow an object to only be shown in wire, shaded, x-ray, etc.

There is a key command for it that you MIGHT have accidentaly typed, or clicked a menu in the vewport shading menu possibly.

Also, the reason why it is not happening anymore, and what leads me to believe that my theory is in some small way true, is you can set your prefs to save viewport settings on a per-file basis, and when you re-load another separate file, the viewport settings are set back to default, or set to NOT save viewport changes at all.

Just a few ideas