Just to bring my comments up to date, I managed to get my car to show the warning at the dealers, and they applied the KIA diagnostic tablet to the OBD port. It showed Fault No B185A, which is 3-way valve open (presumably when it should be shut). We could also see the fault by using the the Built-in diag (see https://www.ksoulev.com/heater_amp_a_c_ ... -2923.html
) where it shows as Fault No 108. The fault does not display on SoulEV Spy or Torque - I guess it is a private manufacturer's code.
The valve was replaced today and, so far, the fault has not returned.