The reason for this would be that the content of the Custom sub tab is unavailable to that particular role
1. Create a custom entity sub tab by navigating Customization > Forms > Subtabs > Click Entity tab. Then, specify the title as Company Profile > Click Save.
2. Create a custom entity field by navigating to Customization > Lists, Records, & Fields > Entity Fields
Name: Company Name
Type: Free Form text
Store Value: true
Applies To: Customers
Display tab >Sub tab: Company Profile
Access> Specify a role (say, Custom Role 1)
3. Navigate to Lists >Relationships > Customers > Edit any customer record > Customize Form (e.g. Custom Customers Form 1) > Subtabs tab > check the Show checkbox for Company Profile sub tab > Save
4. Login as Custom Role 1 and load a customer using the Custom Customers Form 1 and you will see the custom sub tab Company Profile
5. Now login using a different role (e.g. Custom Role 2) than Custom Role 1
5. Go to any customer record and you will not be able to see the Custom tab even though you are using the Custom Customers Form 1
6. The reason is that the content of the custom sub tab Company Profile (i.e. the custom entity field Company Name) is unavailable to Custom Role 2.