N/RPN messages are Control Change (CC) messages. The specification is clear that when CC messages have an MSB and an LSB component, the MSB must be sent first, because it resets the LSB.
If you have found manufacturers that have not followed the spec, then the manufacturer is not in compliance with the spec.
Since NRPN messages are proprietary, it is possible for a manufacturer to choose to only use an MSB value if the parameter being changed does not have more than 127 possible values. You will need to consult the user manual of a device to be certain when to use both MSB and LSB or only MSB.
Cheers
Eddie