Something is wrong with Mail failover after upgrade from 8 to 10 (Mailgun)

[frutality]

We are sending a ton of emails via Laravel Mail. Recently we upgraded framework from 8.83.5 to 10.14.1 following both upgrade guides.

We configured failover configuration like this:

'mailers' => [
        'app' => [
            'transport' => 'failover',
            'mailers' => [
                'mailgun',
                'postmark',
            ],
        ],

        'mailgun' => [
            'transport' => 'mailgun',
            'domain' => env('MAILGUN_DOMAIN'),
            'secret' => env('MAILGUN_SECRET'),
        ],

        'postmark' => [
            'transport' => 'postmark',
            'token' => env('POSTMARK_TOKEN'),
            'message_stream_id' => env('POSTMARK_MESSAGE_STREAM_ID', 'outbound'),
        ],
]

Everything mentioned in this section was considered.

What we noticed is that after deployment this update to production, Postmark sent emails number skyrocketed.

Overall this is not a problem, since all emails are queued, sent and delivered. But this is suspicious and I would like to be sure that everything is really ok. Ratio is around 9:1. Nine emails goes to Mailgun, one to Postmark.

I don't see any related errors in logs. No failed jobs either. Any suggestions what to look at?