GrapheneOS has attempted to cover this, it comes with an Auditor app that can be used from another Android device to check for tampering.
reply> The Auditor app uses hardware-based security features to validate the identity of a device, along with the authenticity and integrity of the operating system. It ensures the device is running a verified operating system with a locked bootloader and that no tampering has occurred." [0]
I assume that it would be quickly discovered if Motorola were tampering with phones en masse.