Finally found time to upgrade to DWC 1.22.6 and can confirm that the behaviour persists.
In summary the "Heaters" section of DWC always shows active and standby temperature for Heater1 as being zero (with a multi extruder, single heater, hot end) .
It could be kind of dangerous as it is possible to set an active tool temperature in the heater section but not know that it has been set because the displayed value is always zero. It's not a problem if temperatures are always set using the "Tools" section of DWC.