See my post for details and a hacky solution.
In the end, did this in the user script until it’s fixed:
if header :regex "To" "(^|[, ]+)([^+]+)\+([^@]+)@([a-z\\.]+)" {
set :upperfirst "folder" "${3}";
fileinto :copy :create "Alias";
fileinto :create "Alias/${folder}";
# notify :message "Uncomment for debugging the variables/${2}/${1}/${0}/${3}" "mailto:[email protected]";
stop;
}