IPSEC VPNs keep logging FIVE terminate/established log entries every hour or so

I have 1 HQ firewall (XG135) and 4 branches (XG105). They all have IPSEC tunnels with the default IKEv2 setup. They are all on 17.5.3.372

 

Spaced about 1 hour 13 minutes apart, I get 5 established and 5 terminated IPSEC vpn tunnel log entries from a branch firewall all with the exact same timestamp. Then a few minutes later, approximately the same hour/minutes apart, I get another 5 established and 5 terminated IPSEC vpn tunnel log entries. A few of the firewalls have TWO IPSEC tunnels, and I'll get an alert on one of the tunnels, and at some point over the next hour, I'll get another alert for the other tunnel, but both don't terminate at once.

 

No internet outages, and no perceived downtime with the tunnels. I have 100 users all using VoIP and no-one has said a word. This has been happening since deployment 2 days ago.

I'm getting bombarded with alerts for tunnel disconnection/re-connection.

What configuration item should I be looking at? Or is this a bug?