The functions that convert regular base64 to url safe base64 terminate their loops on '=' which trims the '=' padding from the final output. While the Azure base64 decoder tolerates the missing padding, it is incorrect and was discovered while testing against a python mock server.

See
https://github.com/mongodb/libmongocrypt/blob/master/kms-message/src/kms_b64.c#L522-L524
https://github.com/mongodb/libmongocrypt/blob/master/kms-message/src/kms_b64.c#L556-L558