Hi
I guess this post explains the behavior. connman somehow gets configured to act as an NTP client along with systemd-timesyncd.
And from this one I guess that has to do with your DHCP server providing a NTP server in its DHCP answer or the default router acts also as an NTP server.
Does disabling connman’s NTP client functionality as suggested in the posts help?
Max
P.S. I’d guess that connman doesn’t agree with systemd-timesyncd on the timezone / daylight saving which makes them constantly setting a different time or the NTP server connman contacts provide a UTC time which is wrong by 3598 seconds.