Is it possible to update the environment from a mutation for a subsequent mutation join?

@jackson.reynolds yes, here is an example:

(ns com.wsscode.pathom.union-repro
  (:require
    [clojure.core.async :as async]
    [com.wsscode.pathom.core :as p]
    [com.wsscode.pathom.connect :as pc]))

(pc/defresolver x [env {:keys []}]
  {::pc/output [::x]
   ::pc/cache? false}
  {::x (:x env)})

(pc/defmutation sample-call [env _]
  {}
  {:foo    "bar"
   ::p/env (assoc env :x "meh")})

(def resolver-registry [sample-call x])

(def parser
  (p/parser
    {::p/env     {::p/reader               [p/map-reader
                                            pc/reader2
                                            pc/open-ident-reader
                                            p/env-placeholder-reader]
                  ::p/placeholder-prefixes #{">"}}
     ::p/mutate  pc/mutate
     ::p/plugins [(pc/connect-plugin {::pc/register resolver-registry})
                  p/error-handler-plugin
                  p/trace-plugin]}))

(comment
  (parser {:x "bla"}
    [::x
     {(list `sample-call {}) [::x]}])
  
  ; => {::x         "bla"
  ;     sample-call {::x "meh"}}
  )