This is definitely bad behavior on the part of UnitConvert, which is mistaking DegreesRankine for a temperature difference unit since it has a fundamental zero value at absolute zero, and a non-zero unit scale (which is what temperature difference units have, but that combination is not exclusive to those units).

I can confirm that this bug has been reported to the relevant developers and an update should be forthcoming to resolve the issue. In the meantime, if you need a work-around setting the following should resolve the warning message and restore the expected behavior for you:

Quantity;
QuantityUnits`Private`hasNonZeroTempUnitQ["DegreesRankine"] = True;