Http-response set-header with condition, not working

I am getting the same error in HAProxy, while using it via the OpnSense firewall.

acl ‘********************************’ will never match because it only involves keywords that are incompatible with ‘frontend http-response header rule’

My case is a bit more weird though, since I try to delete a header :

haproxy

Your previous explanation of the issue does not apply here, I think.
Here, we don’t need any data from the request in order to process the response.

Any idea how I can solve this?
I don’t see OpnSense supporting something related to txn variables and I don’t see how a variable would help in my case.
There is an option for :

Specify a HAProxy rule/ACL that is currently not supported by the GUI.

haproxy1

…but I am not sure what I can do with it.
I guess I could have pasted the:

http-request set-var(txn.path) path

…if I was fixing the original poster’s case, hoping that it works, but in my case I see no benefit in doing that.