Mailgun + WooCommerce: e-mail subject is NULL

The related section of the Log from app.mailgun.com:

{
"tags": [
    "wordpress"
],
"envelope": {
    "sender": "[email protected]",
    "transport": "smtp",
    "targets": "[email protected]"
},
"storage": {
    "url": "https://se.api.mailgun.net/v3/domains/csercsics.hu/messages/eyJwIjpmYWxzZSwiayI6ImVhMGU1YzhjLTY1YmUtNDJlYS1hNDQ5LWI5Yjg1N2E5YzgxMiIsInMiOiI3NjU3MzI2YzIwIiwiYyI6InRhbmtiIn0=",
    "key": "eyJwIjpmYWxzZSwiayI6ImVhMGU1YzhjLTY1YmUtNDJlYS1hNDQ5LWI5Yjg1N2E5YzgxMiIsInMiOiI3NjU3MzI2YzIwIiwiYyI6InRhbmtiIn0="
},
"log-level": "info",
"id": "HsJ3jcNST1m_HMa9-y_hOg",
"campaigns": [],
"method": "http",
"user-variables": {},
"flags": {
    "is-routed": false,
    "is-authenticated": true,
    "is-system-test": false,
    "is-test-mode": false
},
"recipient-domain": "domain.com",
"timestamp": 1517908968.845158,
"message": {
    "headers": {
        "to": "[email protected]",
        "message-id": "[email protected]",
        "from": "Sender Name <[email protected]>",
        "subject": null
    },
    "attachments": [],
    "size": 19249
},
"recipient": "[email protected]",
"event": "accepted"