As there is a bit of speculation ref how the cooling system operates, the following are the basic flow diagrams of the BMW B58 engine ( Plus Six) . These diagrams are from BMW training information available online so only reflect the installation in a BMW vehicle. However, they do show the split flows during warm up and the electronic control valve, Morgan uses broadly the same system but with their radiators, dual expansion tanks ( I believe the BMW expansion tank is a semi twin tank in one housing in some models ) and installation etc.. I believe the Plus Four engine flows are broadly similar. Its a fairly complex system engineered to meet emissions and controled by the ECU. The Morgan ECU reflash must have changed the engine coolant flow control during warm up and maybe at other times of light load, but this is only a surmise.
Dave Plus Six "Squeak" Roadster departed. 1936 BSA Empire Star Q8. 1935 BSA W7-35. 1938 BSA M20