Intermittent issue with fusion rest api while releasing hold on project billing event

Summary:

As part of out OIC orchestration we are first putting hold on project billing event and after some processing we are trying to release the hold.
In both cases we are facing intermittent issue with 'null' error message as below:

null.A 400 Bad Request Error indicates that the target service is unable (or refuses) to process the request sent by the client (Oracle Integration Cloud), due to an issue that is perceived by the server to be a client problem. Try invoking the target service using cURL. It may also be that one of the intermediaries (proxy, LBR) could be returning this code.