Inline task disappears from workbox when editing of page is restricted

I recently had the following scenario:
User A is confluence-administrator and creator of a page called "XY".
User B is confluence-user.
User A sets a page restriction to XY so that only he can edit the page.
User A creates an inline task and mentions User B.

If User B wants to mark the task as done on the page itself he gets an error-message.

If User B wants to mark the task as done in his workbox it disappears from there, which is not the expected behavior. The state of the task does not change on the page.