Windows 11 - Output Preview - Separations

Just noticed an issue when getting a file ready for print. I have a black only job, type and vectors, created in Illustrator and when I save as PDF and look at the separations in Acrobat Pro (most recent version) and turn off the black plate it still shows separations in C M and Y... close the file and reopen and it's fine. This has happened a few times now. Any thoughts, know issues?