I had alt-tabbed away, and then when I came back the game had crashed with the message to please post the log.

Here it is:

0.000 2019-07-12 13:29:02; Factorio 0.17.56 (build 46133, mac, steam)
0.000 Operating system: macOS 10.14.5
0.000 Program arguments: "/Users/alenda/Library/Application ...

I did some further testing based on TruePikachu's observation, and they were correct.

Adding a Roboport closer to that "fault line" than the originating roboport did cause the drones to continue on at their 20% speed setting.

I have to admit, I'd probably have gone on assuming it was a bug. The ...

Hello,

I was experimenting with some debug settings, in hopes of figuring out where all my drones were when I needed them.

Eventually I noticed that many of the drones were caught in this loop.

They'd take off from the robo ports along the train tracks, apparently on their way to a target at a ...