Interval not stable on UniversalTracker sketch

I have the SodaqOne setup to send out data every minute. When I reset the SodaqOne, it does this correctly. After some time the SF goes from 7 up to 12 (while not moving the gateway or SodaqOne) relative to the SF factor going up I also see an increase in the interval. It does not send every minute after some hours passed. At the moment I see data coming in every 10 to 20 minutes.

Maybe this is related to the SF factor steadily going up, like reported in this thread?