I am engaged in a documentary project, recording street and architectural scenes of Phoenix on film. Film will ensure that the images can survive to future when Phoenix may be mostly dust in the wind (after the water supply fails). I geotag the location of each image by pasting GPS coordinates from my iPhone's Google Maps app into an Apple Notes file. After digitizing the developed negatives (with camera scans) I import the files into Lightroom Classic, and copy and paste the coordinates from the Notes file. I don't save the Notes files, but I do place a printed copy of each one in the ring-binder which holds the negatives.
I have been doing this since 2017. There are now about 4,500 images, each with its marker on the Map Module. The icons are crowded; one must zoom in pretty close to differentiate all the individual markers. Two months ago I discovered that one marker would not differentiate, it contained 400 images, each having the same location info. These 400 images were randomly distributed in several dozen folders (I use one folder for each roll of film). I would never have selected these 400 images for any reason, so I could not have unintentially dragged them, as a whole, to a new location on the map. Nor could it be something in the import process -- that would affect every image on a roll, and the GPS contamination is irregular.
I corrected the GPS errors, typing in the correct coordinates from my printed Notes files. That took a few days. Then I noticed two other groups of contaminated coordinates -- one held 80 files with identical GPS, another held 65. I wondered if it might have something to do with syncing. I paused syncing. Now I have discovered a fourth group, containing 365 images. These contain some files which were imported two weeks ago, after I had paused syncing.
Is this a bug in Lightroom? I cannot think of another scenario.
I should add that I store all the files on my hard drive, not in the Cloud.
Here is a screen shot of the Map Module. The 365 images at this location has now been whittled down to 295 after I made corrections to seventy.
The Lost House Key icon has nothing to do with me, or the problem.
"I discovered that one marker would not differentiate, it contained 400 images"
If you look at the GPS coordinates of these images in Library Metadata panel, are they exactly the same or different?
They are all exactly the same.
In the one of grouping of 80 images I remember that the location assigned to all in the group corresponded to the actual location of the first image in the group. The other 79 images were a random assortment, from several folders.
I cannot verify if this was true for the other three groupings mentioned in the OP. If it happens again, I will be on the lookout for this clue.
I haven't seen other reports of similar symptoms over the past many years.
What immediately comes to mind is that you accidentally had 400 images selected when you entered the coordinates for one of them, thus changing all the others too. In general, when all the photos in grid view are selected, the subtle differences in grey shading around the thumbnails makes it easy to miss that they are actually selected. But you wrote, "These 400 images were randomly distributed in several dozen folders... I would never have selected these 400 images for any reason."
Another possibility is that your preferences have been corrupted. You could try resetting LR's preferences:
LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior, including operating on photos that aren't visually selected. That article explains how to restore the old preferences if resetting doesn't help.
Thanks, I will try resetting the preferences, tomorrow when I am fresh.
My preferences are now reset. I will post again if this does not solve the problem.