Unusual pattern in the receiving of packets

I’ve been using chirpstack for a month or so, all is well except an issue which I’ve been experiencing since 4 days. I’ve made a provision in my firmware (LMIC library) to send data every minute, I’ve been receiving data accordingly. But since 4 days, I’m seeing an unusual pattern in the receiving of data - there’s a time gap of 1 minute between 2 consecutive data packets (say t1 and t2), but I’m seeing the next data packet within 6 or 7 seconds (say t2 and t3); again 1 minute between t3 and t4, 6 or 7 seconds between t4 and t5; the cycle goes on. I’m attaching the screenshot of few sample packets, the pattern is evident from the time stamp of the successive packets.

This is also evident from the number of frames received, which we see in the gateway dashboard of the chirpstack app-server. I’m attaching screenshot of the gateway dashboard. Considering the frequency of data transmission every minute, the number of frames per day should be 1440. As we can see, on 25th Jan 2020, the number is way above 1440; and on 28th Jan 2020, where I had collected data all 24 hours of the day, it was 2472 (there were some issues on 26th and 27th Jan, so there was a break in data collection and hence the number of frames is less).

How do I fix this? Where does the problem lie - Device, Gateway or Network Server?

I’m using Adafruit Feather M0 with LoRa as my end-node, and IMST+Pi3 as my gateway.