I have tried to reproduce the issue with DEBUG logs enabled, but for some reason the folder has been syncing properly now 
. I paused everything except the two involved devices and the one folder, and then tried to re-add it from scratch as receive only on the second device, but this time the files synced properly.
The only issue experienced while doing the testing was the local additions problem discussed in Folder stuck in sync and non-matching local and global states, for which I do have proper logs now. However, the log files are massive (400MB and 800MB respectively) and require anonymisation, so I’m not sure when or if I will be able to do something about them.