After reboot, I see this in pm2 status:

pi@mm1:~ $ pm2 status
┌─────┬────────────────┬─────────────┬─────────┬─────────┬──────────┬────────┬──────┬───────────┬──────────┬──────────┬──────────┬──────────┐
│ id  │ name           │ namespace   │ version │ mode    │ pid      │ uptime │ ↺    │ status    │ cpu      │ mem      │ user     │ watching │
├─────┼────────────────┼─────────────┼─────────┼─────────┼──────────┼────────┼──────┼───────────┼──────────┼──────────┼──────────┼──────────┤
│ 0   │ MagicMirror    │ default     │ 2.12.0  │ fork    │ N/A      │ 0      │ 0    │ stopped   │ 0%       │ 0b       │ pi       │ enabled  │
└─────┴────────────────┴─────────────┴─────────┴─────────┴──────────┴────────┴──────┴───────────┴──────────┴──────────┴──────────┴──────────┘

When I check the pm2 log, I see:

2020-10-31T16:18:53: PM2 log: [PM2] This PM2 is not UP TO DATE
2020-10-31T16:18:53: PM2 log: [PM2] Upgrade to version 4.5.0
2020-10-31T19:29:27: PM2 log: App [MagicMirror:0] exited with code [134] via signal [SIGINT]
2020-10-31T19:29:27: PM2 log: App [MagicMirror:0] starting in -fork mode-
2020-10-31T19:29:27: PM2 log: App [MagicMirror:0] online
2020-10-31T19:30:01: PM2 log: Stopping app:MagicMirror id:0
2020-10-31T19:30:01: PM2 log: App [MagicMirror:0] exited with code [0] via signal [SIGINT]
2020-10-31T19:30:01: PM2 log: pid=9716 msg=process killed
2020-10-31T19:30:01: PM2 log: PM2 successfully stopped
2020-10-31T19:32:10: PM2 log: ===============================================================================
2020-10-31T19:32:10: PM2 log: --- New PM2 Daemon started ----------------------------------------------------
2020-10-31T19:32:10: PM2 log: Time                 : Sat Oct 31 2020 19:32:10 GMT-0400 (Eastern Daylight Time)
2020-10-31T19:32:10: PM2 log: PM2 version          : 4.4.1
2020-10-31T19:32:10: PM2 log: Node.js version      : 10.21.0
2020-10-31T19:32:10: PM2 log: Current arch         : arm
2020-10-31T19:32:10: PM2 log: PM2 home             : /home/pi/.pm2
2020-10-31T19:32:10: PM2 log: PM2 PID file         : /home/pi/.pm2/pm2.pid
2020-10-31T19:32:10: PM2 log: RPC socket file      : /home/pi/.pm2/rpc.sock
2020-10-31T19:32:10: PM2 log: BUS socket file      : /home/pi/.pm2/pub.sock
2020-10-31T19:32:10: PM2 log: Application log path : /home/pi/.pm2/logs
2020-10-31T19:32:10: PM2 log: Worker Interval      : 30000
2020-10-31T19:32:10: PM2 log: Process dump file    : /home/pi/.pm2/dump.pm2
2020-10-31T19:32:10: PM2 log: Concurrent actions   : 2
2020-10-31T19:32:10: PM2 log: SIGTERM timeout      : 1600
2020-10-31T19:32:10: PM2 log: ===============================================================================

I can start Magic Mirror manually by issuing:

pi@mm1:~ $ pm2 restart MagicMirror
Use --update-env to update environment variables
[PM2] Applying action restartProcessId on app [MagicMirror](ids: 0)
[PM2] [MagicMirror](0) ✓
┌─────┬────────────────┬─────────────┬─────────┬─────────┬──────────┬────────┬──────┬───────────┬──────────┬──────────┬──────────┬──────────┐
│ id  │ name           │ namespace   │ version │ mode    │ pid      │ uptime │ ↺    │ status    │ cpu      │ mem      │ user     │ watching │
├─────┼────────────────┼─────────────┼─────────┼─────────┼──────────┼────────┼──────┼───────────┼──────────┼──────────┼──────────┼──────────┤
│ 0   │ MagicMirror    │ default     │ 2.13.0  │ fork    │ 792      │ 2s     │ 0    │ online    │ 0%       │ 2.4mb    │ pi       │ enabled  │
└─────┴────────────────┴─────────────┴─────────┴─────────┴──────────┴────────┴──────┴───────────┴──────────┴──────────┴──────────┴──────────┘

Thanks!