Syncing happening but, wrongly

Omnifocus is failing to sync actions across devices that have been completed.

I’ve, just now, completed some actions on my iPhone that are shown as incomplete on my Mac. From Omnifocus on the Mac [> Preferences > Sync > Show Sync Details…], the times for syncs are up-to-date. If I force a new sync from each device, they show up but the actions remain out-of-step. I.e., it doesn’t look like a failure to sync; the problem is that the information from the sync is failing to get through.

If I close Omnifocus on the Mac, and re-open the actions are, now, correctly shown as complete.

I often (a few times a week) find that one or two actions have become duplicated too. Perhaps a symptom of the same problem…?

I’m pretty sure that this has been happening for quite a while but today is the first time that I’ve carefully investigated it.

Is there a problem with the Omni Sync Server or is there something on my devices that I need to do?