Why was I force-resigned from a 1-day-old casual unlimited game against the computer?

I started this game yesterday: lichess.org/I6NV3hTiCryz — but this morning the site says I resigned, but I did not resign.

The only out-of-the-ordinary thing I did was to start the game on the Web site but my last interaction with it was from the Android app.

This has happen several times before, though I can't remember if I switched between the Web/mobile interfaces on those games too.

It happened again. I was force-resigned from this not-yet-1-day-old casual unlimited game against the computer:

This time I did NOT interact with this game from the mobile app. Just started the game last night, and I find myself resigned today.

I'm using Firefox 64-bit (fully updated) on Windows 7 and 10. I started the game on Windows 7, and found myself force-resigned when I went to the game on Windows 10 the next day.