HomeKit thermostats and Celsius Since upgrading to iOS 13 and macOS 10.15, one can only set temperatures in an integer number of degrees, which works fine for Fahrenheit, but isn't fine for Celsius. iOS 12 and macOS 10.14 permitted half-degree values when using Celsius and integer degrees in Fahrenheit.

Aside from creating/editing scenes with explicit temperatures on a Mac running 10.14, does anyone have a suggestion for a solution?
There's no way to make that change directly in the Home app.  This issue was mentioned during the iOS13 beta testing and wasn't changed.

Some people have reported that as of iOS 13.1.3 you give a command to Siri and it will set the correct temp.  I'm in the US, thus don't use celsius, so I can't confirm this is correct.  You didn't mention which specific iOS 13 version you were using.

Apple's 13.1.3 release notes don't specifically call this out.  It's not unusual for Apple to fix things and not put them into the release notes.