This is not a bug in either implementation. There is no requirement to escape U+00B0. To quote the RFC:
2.5. Strings
The representation of strings is
similar to conventions used in the C
family of programming languages. A
string begins and ends with quotation
marks. All Unicode characters may be
placed within the quotation marks
except for the characters that must be
escaped: quotation mark, reverse
solidus, and the control characters
(U+0000 through U+001F).Any character may be escaped.
Escaping everything inflates the size of the data (all code points can be represented in four or fewer bytes in all Unicode transformation formats; whereas encoding them all makes them six or twelve bytes).
It is more likely that you have a text transcoding bug somewhere in your code and escaping everything in the ASCII subset masks the problem. It is a requirement of the JSON spec that all data use a Unicode encoding.