Hello @Daniël Pastoor,
Thank you for contacting Microsoft!
I have created a team's channel with isFavoriteByDefault property set to true, which can only be set programmatically while creating teams using the below link:
I have added new members to the team, and the channels are functioning correctly. The channel with 'isFavoriteByDefault' set to true is displayed as expected to the new members. I recommend creating the team channel with this property, following the example given in the document above.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment.