Hello,
Teltonika devices are manufactured to be primarily operating with the Teltonika software and bootloader is also a component of this mindset.
In general, it is not recommended to change the bootloader software at all, and even in some individual cases it is only recommended to upgrade it, as there is a possibility to brick the device. Different bootloader version add new hardware support, which occasionally changes between the batches. Thus, the downgrade will most certainly result in device not being able to start properly without the possibility to recover it.
Best regards,
Žygimantas