In addition to androids power management, some vendors ship an additional pm to make our life harder.
Because one is just not enough.... 🥁 ...Huawei comes with two 🎉

Powergenie (pid = 3920 in the log below) seems to kill Briar now and then:

I/ActivityManager: Force stopping org.briarproject.briar.beta.debug appid=10113 user=0: from pid 3920by app
I/ActivityManager: Killing 31369:org.briarproject.briar.beta.debug/u0a113 (adj 2): stop org.briarproject.briar.beta.debugfrom pid 3920by app

From @Wikinaut and my observations this happens when the wifi connection is lost while Briar is in the background and the screen is off. I was able to reproduce this a couple of times:

1. Start Briar
2. Turn off wifi
3. Wait for some time