Open rates freeze and other stats are not updating

I run using Ghost 4.8.4. From time to time, I observed that emails’ open rates stopped updating (in Mailgun logs I was seeing updates that were not reflected in the Ghost admin console).

Today the situation happened again, and the open ratio stuck at 3% (after a quick check, it should be > 30%). What’s more, for several members, I noticed that the “EMAILS RECEIVED” value was not incremented even though I’ve seen delivery status in Mailgun logs.

Similar problem reported recently:

Can anyone help here?