With everything off, I ran an action which sets the fan to 'always on' and verified that it was successful at the thermostat. I then ran an action that set the temperature to 20C, and verified that the fan mode was now 'auto on'.
In my Ecobee account, I have 2 thermostats at 2 homes. The one at interest here is labeled "stairs"; ignore "Front Hall" and "North Office".
Sep 16, 2020 at 12:16:09
Ecobee 2 Debug logLevel = 10
Ecobee 2 Debug updateFrequency = 900.0
Ecobee 2 Debug setting temperature scale to C
Ecobee 2 Debug Ecobee Account (1345636757): Ecobee Account Update OK, got info on 2 thermostats
Ecobee 2 Debug Ecobee Account (1345636757): getting data for 'Front Hall', 521703550260
Ecobee 2 Debug Ecobee Account (1345636757): getting data for remote sensor 'North Office', C5SW
Ecobee 2 Debug Ecobee Account (1345636757): getting data for 'Stairs', 521781046633
Ecobee 2 Debug Ecobee Stairs: Updating device
Sep 16, 2020 at 12:17:57
Action Group Fan ON-60min
Ecobee 2 Debug Ecobee Stairs: action.thermostatAction: SetFanMode
Ecobee 2 Ecobee Stairs: set fan to ON, leave cool at 26.1 and heat at 7.2
Ecobee 2 Debug API 'set hold temp with fan on' request completed, result = <Response [200]>
Ecobee 2 Debug Ecobee Account (1345636757): Ecobee Account Update OK, got info on 2 thermostats
Ecobee 2 Debug Ecobee Account (1345636757): getting data for 'Front Hall', 521703550260
Ecobee 2 Debug Ecobee Account (1345636757): getting data for remote sensor 'North Office', C5SW
Ecobee 2 Debug Ecobee Account (1345636757): getting data for 'Stairs', 521781046633
Ecobee 2 Debug Ecobee Stairs: Updating device
Sep 16, 2020 at 12:19:10
Action Group htrSetpt_20C
Ecobee 2 Debug Ecobee Stairs: action.thermostatAction: SetHvacMode
Ecobee 2 Debug Ecobee Stairs (521781046633): Mode set to: auto
Ecobee 2 Debug API 'set HVAC mode' request completed, result = <Response [200]>
Ecobee 2 Debug Ecobee Stairs: actionActivateComfortSetting
Ecobee 2 Debug API 'set climate hold' request completed, result = <Response [200]>
Ecobee 2 Debug Ecobee Account (1345636757): Ecobee Account Update OK, got info on 2 thermostats
Ecobee 2 Debug Ecobee Account (1345636757): getting data for 'Front Hall', 521703550260
Ecobee 2 Debug Ecobee Account (1345636757): getting data for remote sensor 'North Office', C5SW
Ecobee 2 Debug Ecobee Account (1345636757): getting data for 'Stairs', 521781046633
Ecobee 2 Debug Ecobee Stairs: Updating device