The current behavior of the BSON decoder is to apply the rules for uuidRepresentation whenever it decodes a BSON Binary with subtype 3 or 4. It should ignore uuidRepresentation when the subtype is 4.
This bug only applies when the application has set uuidRepresentation to javaLegacy or csharpLegacy, but the database returns BSON Binary subtype 4 (standard). This should be very rare. standard and pythonLegacy (the default) use the same byte order.
This is a minor backward breaking change in behavior to fix a bug. We'll put a warning in the changelog.