LMiC implements a safety delay in which it assumes the gateway was actually sending a downlink for the node in RX2, but the node just failed to pick it up. The idea is that in that case sending too soon makes no sense as the gateway won’t be listening.
Earlier versions of LMiC would even delay EV_TXCOMPLETE for that; recent versions still have that delay before one can transmit again, but fire the EV_TXCOMPLETE after about 2 seconds.
So: is LMiC up to date?