Normal Tech accounts that are under a tech permission that is Limit to Assigned Tech Groups can only see the request type assigned to their Tech group (which is fine)
But they should be able to see the sub request types that are assigned to them. On 12.6 this is no longer the case because if the PARENT RT is assigned to a different Tech Group or is left blank then they cannot see that RT>sub RT at all when trying to raise a ticket. On 12.5.2 You can leave the Parent RT blank and all techs can see this.
Parent Request Type - Tech Group: BLANK
Child Request Type 1 - Tech Group 1
Child Request Type 2 - Tech Group 2
Normal Techs under Tech Group 1 and 2 will not be able to see Child Request Type 1 and Child Request Type 2