The ghost of owner past

Hi all, I’ve just bought a superb estate, registered December 2015. 
The previous owners Bluetooth profile was still on the car when I drove away, I deleted it, paired my own Apple iPhone 10 and thought nothing of it. When restarting the car, my Bluetooth pairing has been forgotten and the previous owners is back again. I then went to settings, factory reset, reset all settings. Hoping I would erase it for good, pairing my device again after. The following vehicle restart my Bluetooth has been erased and the previous owners back!

Has anyone experienced this in the past?
Are there any other types of memory erasing I can do to wipe the systems memory?