Photos sync to iCloud stuck - possible strange conflict with Dropbox?

I recently had an issue with Photos stuck not syncing to iCloud from iPhone.

After a couple of days of trying various steps I found that Dropbox had for days been stuck syncing a very large file from my MBP on the same network (same iCloud). Bizarrely, the second I paused Dropbox syncing, the iCloud photos syncing started up again on my iPhone.

There was plenty of bandwidth left on the connection for video streaming etc.

You don’t need to tell me it could be coincidence. I’m posting here to see if this had happened to anyone else and is therefore not a coincidence.

Anyone else noticed this issue?