You cannot add attributes in the way you are doing so currently. You need to add the target attribute using a full SCIM URN, as outlined in https://datatracker.ietf.org/doc/html/rfc7643#section-10.2.1
The specific example I am referring to is:
Declaration of Syntactic Structure:
The Namespace Specific String (NSS) of all URNs that use the "scim" Namespace ID shall have the following structure: urn:ietf:params:scim:{type}:{name}{:other}
Ideally you should not be seeing this work in any scenario, but the fact it is working at all is an unintended thing due to the fact that the code for us generating a POST is different than the code where we generate a PATCH. Your current attribute additions are not supported as they are not using a SCIM-compliant attribute naming convention.