Temperature Measurement calculations error

I've found the following inconsistency in how the temperature Measurement addition seems to work.

If I add two like temperatures:

let t1 = Measurement(value:25, unit: UnitTemperature.celsius)

let t2 = Measurement(value:10, unit: UnitTemperature.celsius)

print(t1+t2)

I get the expected 35.0 °C. However with unlike temperatures:

let t1 = Measurement(value:25, unit: UnitTemperature.celsius)

let t2 = Measurement(value:10, unit: UnitTemperature.fahrenheit)

print(t1+t2)

I get 559.07777777778 K or 285.92777777778 °C.

I realize that adding temperatures is problematic, but these results are not consistent.