Photos turn black and white after opening a folder

Thanks for this.  I have taken a look at both threads.  The first relates to previews, whereas my problem is that the photo is black and white both in Library and Develop.  (In Develop, "Black & White" is active in the "Treatment" line.)  To answer your question about what I see in History, it's typically the import and then "Synchronize Settings," which is what I would expect to see because after importing I select "Enable Profile Corrections" and sync that to all photos in that date folder.

The second thread relates to photos turning black and white on import.  That is not happening on my case.  (I have just now imported today's photos to verify this.) 

Two more observations from further experimentation:

(1) The black and white effect does not occur on date folders I am opening from about mid-October 2020 onwards.  I have also opened a couple of date files from early this year, but in a part of my catalog outside where I rearranged the folders, and there is no problem with those. 

(2) When I change photos in a date file back to color by switching to "Color" in Develop and syncing "Treatment & Profile" to the other photos, that change sticks.  I have closed and reopened LR and those photos are and remain color photos when I open those particular date folders.

Most of the time when something goes wrong in LR or Photoshop, it's my incompetence and I just need to take a bit of time to work out what I'm screwing up.  This problem, though, is so bizarre that I wonder whether it falls into the "Well, sometimes Lightroom and Photoshop malfunction for no good reason and do crazy stuff" category.   The one piece of good news here is that I have a Lightroom catalog backup that is only one week old, and I think I could replicate the work done since then (importing photos and then the folder rearrangement) in an hour or so.  So perhaps the best thing here is to work out how to instead use the backup catalog and, assuming that it doesn't exhbit the same problem, replicate the work since then, hoping that it's problem-free next time around.  Does that make sense?

Sorry, yes, I meant the right "year" folder.  I moved the "date" folders inside Lightroom by dragging them into the correct "year" folder on Lightroom's left side panel (except for a few strays that were in the wrong place on my hard drive to start with, which I moved outside LR and then imported into LR).  When I add photos to LR, I use "Import" (usually straight from the memory card), not "Folder Synchronize."  The affected photos were imported long ago.  I have no idea whether the folder reorganization is relevant to the black and white problem, but I mention it because it was the last thing I did before the black and white problem started, so it's possible that these are related.

No, I am not accidentally touching the V key.  I merely open a date folder and then watch the photos turn black and white one by one while I am not touching the keyboard or doing anything else.  I nonetheless checked that my V key is not stuck, although even so I don't see how this could affect all the photos in a date folder given that I have not done a Select All, let alone a Select All followed by Sync when I open each date folder.  I also watched in the upper left as the photos were turning black and white and could not see the progress bar indicating a sync.

I have done a bit more experimetation.  The following types of photos do not turn black and white:  (1) photos taken on my iPad or iPhone that I added to my PC's hard drive and then imported into LR and (2) TIFs, being the photos I have edited in Photoshop.  Therefore, the photos turning black and white are those photos taken on my camera and not edited in Photoshop (which are DNG in LR, as I convert to DNG when I import). 

Thank you for your good questions.  This is the most mysterious problem I have ever struck in LR!

Believe me, that was not my intention.  There was a spelling error in the first one.  Couldn't work out how to edit it.  So figured I would post another and then delete the first.  I have lost about 15 minutes today that I will never get back researching how to edit and delete posts in this forum to discover that neither is possible.  Never crossed my mind that the manufacturer of the most sophisticated photo editing software out there wouldn't provide such basic functionality in its online forum.