I am a long time user of PM. I am having the same issue. Except the slide show hangs up when I select tagged thumbnails, then select all, then run PM slideshow. The slide show only runs a few images. The problem seems related NIKON Z7 NEF after recent FW 2 upgrade. If I do not select tagged images, the slideshow works.
On another note: If I select tagged images from a folder shot with Nikon D5, the slide show works
My problem went away when I uninstalled PhotoMechanic6R3023 version and went back to version PhotoMechanic6R2818.
I may send in a support ticket to have this researched further.