It seems that the JSON default encoding is UTF-8 and Spring MVC returns application/json; charset=utf-8 by default, which is hard to change.
Returning a charset parameter on application/json is incorrect, as that media type does not define a charset parametert. See https://greenbytes.de/tech/webdav/rfc8259.html#rfc.section.11, last sentence:
Note: No "charset" parameter is defined for this registration. Adding one really has no effect on compliant recipients.