TLDR: Device OPC server time needs to be correct if using On Change.
Hullo peeps,
Quick followup for anyone finding this!
After a chat with support it was found that the OPC server time in the PLC that it was drawing from was wrong. When using 'On Change', the historian now takes the timestamp from the OPC server. The ability to use the local timestamp when using 'On change' was removed as per the below from Colby Clegg.