@Matthijs van Mulligen - This is by design, presumably because the initial use case from which this feature was designed was to allow a user to share their templates, but not have people change their templates. For more information, have a read of the Custom Templates help center article.

If you would like the ability to allow others to edit your templates, I would suggest adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know. If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.

@Robert Johnson Thanks for the information. Hereby I would like to contribute to new Miro features with sharing that idea.

I posted it in the Wish List: https://community.miro.com/wish-list-32/template-editing-by-fellow-editors-2986?postid=12525#post12525