Lightroom Classic changes my GPS data

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.