This is new to me as of today. This is a process I have done hundreds of times in the past with no issue. I have not received or updated anything over the past week other than any Adobe updates I receive via the Creative Cloud.

What I do is create a separate layer in a file to set up a kiss cut for cutting vinyl. I use a spot colour for this - same spot colour I have been using since day one.

I apply the spot colour to the text or object - in this case a box with rounded corners. There are other objects - outlined text - that have the same effect applied to it.

I created the box separately.

I select everything on the layer I want to use to create a kiss cut for.

I apply a 1 pt stroke to everything. I open the Attributes panel and set the stroke to overprint.

I go to the Separations Preview and select Overprint Preview.

Everything on the kiss cut layer shows that it is overprinting correctly except the box. All of the outlined text is fine - only the box is not behaving as expected.

I quit Illustrator.

I reopen Illustrator.

The box now appears to be behaving correctly and shows that it is overprinting properly.

I am working on an iMac, Retina 5K, Late 2015, 27-inch screen

3.2 GHz Intel Core i5
24 GB 1867 MHz DDR3 RAM
AMD Radeon R9 M380 2 GB graphics card.

Quitting and reopening is not an ideal solution.

As mentioned, I've not had any issue until today.