I tried more to reproduce this bug, the problem only affect linux 4.19.0-0.bpo.4 and 4.19.0-0.bpo.5 (while I haven't tried earlier bpo versions), and does not affect 4.9 or 4.19.0-0.bpo.6
Since the affecting surface is small so far, I would suggest anyone run into this problem on stretch(-bpo) to upgrade to 4.19.0-0.bpo.6, it does not matter whether the kernel is the current running one at the time wireguard.ko is built. Cheers, Aron
