When you remove a user from a project or from the HUB, the issues that were assigned to that user keep their current assignment. When removing a user from a project/hub it is a good practice to immediately reassign his issues using the search filters and the multi edit option available in the issue tab on the website. See the images below for a step by step procedure for multiple issues edition.
1. Open the issue tab.
2. Click the "Filter" button to expend the filter menu.
3. In the filed "Assigned To", select the user that was removed from the project/Hub.
4. Press the "Refresh" button.
5. Click the checkbox at the left of the "Title" column to select all the issue filtered.
6. Locate the gear icon at the top left and from the drop down menu, select the option "Multi Edit".
7. In the "Edit Configuration" window, select the new user that the issue will be assigned to.
8. Press the "Save" button and all the issues will now be re-assigned to that user.