Ok, after debugging the request following this blog it turned out that the parameters were being sent as a string "parameters":"{\"par1\":\"par2\"}" not matching the jsonb format.

I just changed the declaration of parameters in ProductFormDTO from

private Map<String, String> parameters;

to

private String parameters;

and deserialized it using ObjectMapper in ProductFormToProductEntity mapper

ObjectMapper objectMapper = new ObjectMapper();
productEntity.setParameters(objectMapper.readValue(productFormDTO.getParameters(), new TypeReference<Map<String, String>>() {}));