I'm experiencing exactly the same. SPI communications with the iMST seems to be OK as no errors are logged. The symptom is that I do not receive any packets anymore. Simply restarting the ttn-gateway process "solves" this issue.
I may also try to get a better case, and have a look at the receptions statistics: if no packets are received within some set time interval restart the packet forwarder. Not nice, but better than missing lots of packets.