Thumbnails in library converting to B&W on their own.

A long time ago, I wanted to see what a batch of images would look like in b&w, so I used the V key to do a quick convert in the Library Grid.  From then on, random lines of thumbnails (only in that particular folder) would automatically change to b&w in-front of my eyes.  I can press V again to return to colour, but they still go to b&w.  I can scroll up and down and it will just start in another location.  The files are DNG files, I'm using Lightroom Classic CC 7.5 on a 2007 iMac with OS10.11.6, I cannot update it any more.  I can't tell you what version on LR it is as since writing this, it will not open and I am constantly Force Quitting.  It's really irritating and any advice would be gratefully recieved.