After creating a new build and matchmaking queue, the first several matchmaking attempts often fail after matching with other players, with "Cancellation Reason: Internal".
We upload a new build, often with 1 standby and 1 max server, and then setup a matchmaking queue to point to this build.
Sometimes increasing the standby count or re-uploading the build, or re-creating the queue, or deleting and re-adding regions fixes the issue, but it seems quite random.
I have checked that the 1 standby server has successfully called ReadyForPlayers and is on standby.