Filtering by treatment to show B&W images finds only some B&W

I have many B&W images, all as virtual copies and have found the both filtering by treatment and as a smart collection on this same setting only finds some of my B&Ws.  I've checked several of those missing in the filtered group, and are all set in Develop to Black and White, profile Adobe Monochrome.

For one of the missing B&W virtual copies, I made a new virtual copy, and this was missing too in the filtering.  So I took it into Develop made one change - Treatment to Color then back to B&W.  This new image was now 'found' in the filtered group. Any ideas on how I can fix this nasty issue please?

Using Lightroom Classic v11.2 under Windows 10.