I’ve experienced this a couple of times and found that if I leave it, it will eventually complete though it can sometimes take nearly two hours before the update process kicks in and starts (Obviously, Maintenance Mode will disable itself). We’re running on-premise (VM). I did noticed the update job often appears to be waiting for another queued job to complete.
I’ve raised support tickets about this, but no solution has been provided. The only consistent factor I’ve observed is that we always shut the system down and take a VM snapshot beforehand. Interestingly, when I run an upgrade without creating a snapshot, the process begins immediately, so something is causing it after it starts up.