Yep: it was an ignore file that I had setup and forgot about on two original machines and had not yet setup on the third one. Specifically, I am using the files from here: Useful .stignore Patterns I’ve set it up on the third machine and all is great now. Thanks for your help guys!
Before asking for help, I had looked at the GUI on all three machines and the log files, and launched syncthing in verbose mode. When looking for ignores, my first reflex was to search for them in the GUI even in the advanced settings. I went to the command line only at the end. If you feel this issue with ignores comes up often, maybe having a way to see in the GUI if one is set could be helpful. It would have been for me.
Cheers