Secondary actions in adaptive cards broken for channel conversation

I have a strange behaviour in teams where an adaptive card is rendered differently depending on if the bot is posting it in a chat or in a teams channel.

The adaptive card (reduced to the absolute minimum):

{
  "contentType": "application/vnd.microsoft.card.adaptive",
  "content": {
    "type": "AdaptiveCard",
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.4",
    "body": [
      {
        "type": "TextBlock",
        "text": "Intro text"
      },
      {
        "type": "ActionSet",
        "actions": [
          {
            "type": "Action.Submit",
            "title": "Set due date"
          },
          {
            "type": "Action.Submit",
            "title": "Secondary option",
            "mode": "secondary"
          }
        ]
      }
    ]
  }
}

 When the bot sends it into a chat, the rendering of the secondary action looks like that (expected behaviour):

When it sends a message into a channel, the secondary option is rendered like that:

There is a rectangle in front even though there is no image specified. Adding an icon to the action makes it even worse. The icon is not rendered properly but totally blown out of proportion.

Is this a bug? Is there maybe a parameter that would prevent this rendering?