Permission issues - participant can't asign values to custom fields

Please read my post again. We were talking about fields, not user accounts.

This is not true. A user that is not part of the project and is then made a participant (not assigned, just added at the very bottom, don’t confuse that please) in the task, can see everything and even change a lot of things like the description. However, he can’t edit the custom fields.

I’m quite sure you did not understand the issue here. Please read my post again. The answer does not in any way make sense.

I’m talking about a set of permissions A given by project membership and a set of permissions B given by being a participant of a task. However, in Asana, permission set A annihilates permission set B and the user loses permission, when the intention is to give him more permissions.