Sometimes no downlink ack sent by NS

My nodes are working with latest NS(3.9.0), in most cases, the uplink and downlink messages work very well:

But sometimes the uplink message is not acked by the server, then the node have to resend:

Sorry, new users of this forum can only post one image everytime.

The frame looks normally:

This problem is random, the trace is listed below, can’t see any error:

Does anybody meet such issue? Not sure whether it’s a bug of NS.

Root cause found. There’re two Lora GWs with the same MAC, I believe it confuse NS’s scheduler strategy. After remove one GW, issues disappear.