@GraemeBrowning_Aurizon I believe this is unexpected behavior. Recommend to trace back the configuration of the networks and the route loading process.
Conceptually, LRS data model allows you to have multiple LRS networks. After configuration on the LRS networks, you should verify the dLRSNetworks domain.
'The Network ID field is registered with the dLRSNetworks coded value domain when the LRS is created.'
After, Append Routes or Create Routes verify the Centerline Sequence table records.
'The combination of the network ID and route ID creates a way of uniquely identifying each route in the LRS. The use of the network ID helps differentiate between LRMs, as route IDs may not be unique across networks.'
If you generated Calibration Points, verify the Network ID field as well. If appending Calibration Points, proper Network ID values should be assigned.
'Calibration points are specific to an LRS Network and comprise the measure component of the LRM.'
Ayan Palit | Principal Consultant Esri