Custom Permissions exported as fixtures remove Original Permissions for a `DocType`

ERPNext
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  1. I create two new roles.
    Role a and b.
    I export both of them as fixtures that are saved on role.json.
  2. I need to give role a and b permissions for an existing DocType c. I use the Role Permissions Manager page to do the same. DocType c has permissions for existing role d and e. After adding permissions for role a and b, DocType c has total 4 rows in Role Permissions Manager. I export the permissions for role a and b which are stored as custom_docperm.json.
  3. I reinstall the site.
  4. When I go to the Role Permissions Manager, it has only 2 rows. The existing permissions for role d and e are not shown. Only permissions for role a and c are shown.
  5. I need to see 4 rows instead of 2.
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@rmehta can you help me with this?