Answer
Open the custom field's own window and click its Presentation Object link (shown as a blue hyperlink), then RMB > Manage Grants > Presentation Object Grants, and check the Users tab to confirm the affected users are actually listed there. Creating a custom field can introduce its own presentation object that needs to be explicitly granted, separate from whatever the users' existing permission sets already covered for the underlying screen — adding the missing users to that grant restores their access.