This is really interesting
In terms of sending things for repair and the company saying it all works fine. I had a similar experience with Asus and a ZenFone 9 phone. I bought the UK version, and sent it back for repair. I provided a video with the issue and they said that they couldn't replicate the issue but as a gesture of good will they would repair the motherboard and guess what? That solved the issue!! Plenty of people had the same issue and several firmware updates were released and NONE solved the issue. For the phone it was easy to see because the IMEI and MAC addresses changed. So, given that the Walkman is an Android device , maybe you can write down the Mac addresses (Bluetooth/WiFi) and then compare against the "repaired version". If it's an authorised Sony repair centre it should all be ok. They're "embedded" to the motherboard so if that is actually changed then MAC addresses should (in theory) change with it.