Receive only folders are send-receive folders, that just behave differently while scanning (sets a receive only local flag) - so it should work exactly the same. And the setting is an early abort during the syncing operation, local flags don’t come into that. So essentially it should work and no clue what’s going on. Logs with model debug facility on and configs may provide some clue.
That screenshot is fine, it shown the relevant stuff (ID matches log and ignore delete is set).
Unfortunately the log doesn’t help, it’s just a sign of everything working as intended (it’s not being deleted) - however it is. You could share the full log, even if not directly related there is a minimal chance there is something noteworthy in there. Other than that the only idea I have is to run a tool like fatrace that logs all filesystem operations, thus we can find out when it is deleted and check the logs for what Syncthing is doing at that time.