Hi So Inoue,
This seems indeed to be a Mendix bug I suggest you report this at support.mendix.com.
I think the easy fix is to rename the hidden variable name: if you open the 'Call REST Service' activity, you can set 'Response handling' in the Response tab to 'String' for example and then rename the variable. If you then set the response handling back to your desire, the issue should be resolved without affecting the response handling.
Good luck!