Custom text for http status header being overwritten

I have a legacy application that I am trying to migrate and utilise the features of Istio. However a problem that I have noticed is that Istio appears to be overriding the custom text being set in the status header by the application, for example

Instead of returning “201 Created - Partially Successful” which is what is specified by the application the response being received is “201 Created” which is the http default for that status code.

I believe it is the envoy layer which is performing this operation so is there anyway to intercept and prevent the custom text from being removed?

EnvoyFilters are able to set or change headers. But it’s variable [":status"] only accepts numbers. So it might not be possible.

Thanks I had been investigating EnvoyFilters for this, but similarly could not see the text displayed through the [":status"] header