I think it’s more an iOS 13 bug than an OpenHAB bug. Maybe someone has the same issue, that Siri answers for example 21°C as -6°F but in the HomeApp the temperature is displayed as 21°C like it should be. In my HomeKit configuration “Use fahrenheit” is set to false.
On your iOS device check
Settings ->General -> Language & Region -> Temperature Unit
There is everything correct
i found a german site on which this problem is also listed: