Kong not sending base64 encoded body data to upstream


We have been using the Kong API gateway and everything seems fine except kong is not transferring a PATCH request to an upstream server if there is a base64 encoded big-sized data in JSON payload. It used to work last year and stopped working suddenly and always returns 500 now. There is no incoming request I see related to this PATCH request on the upstream server. It works as soon as I remove the base64-encoded data field from JSON. So, not sure if I need to add any plugins or settings. I tried adding the " Request Size Limiting" plugin, thought maybe it is not allowing that big content-length and set it to 25000 MB, and still not working.
Any suggestions plz?