I am facing the similar issue, I have renamed the app from everywhere but still the on Testflight it's showing the older name only. I have even created the new external tester group and added the build with the updated app name only, but nothing works.

What have been tried -

1. Updated the app name from everywhere in project settings
2. Updated the app name from App Information section in AppStoreConnect.
3. Created the tester group with updated build only.

Please let me know if anyone have already done ?