In the Frontend version of the thermostat, the temperature step-size is based upon target_temp_step. If this parameter in not defined then it derives the step-size from the unit system. For Fahrenheit it uses 1 and for Celsius it’s 0.5.
In the Lovelace version of the thermostat, there’s no code for setting the roundslider’s step parameter. step is defined here and its default value is 1.
Ideally, it should configure itself like the Frontend version and use target_temp_step. If not defined, it should auto-configure itself based upon the unit system (metric=0.5, imperial=1.0).