cider.nrepl.middleware.pprint/fipp-pprint is available in the repl (evaluates to a function), so the middleware should be loaded too (and it works fine in the repl itself)

What cider version?

[cider/cider-nrepl "0.18.0"]

Hmm, I wondered if it was the buggy version of cider which had unreliable printing

It isn't

I don't suppose you have other middleware?

My whole profile is

{:user {:plugins [;; ultra pretty prints stacktraces
                  [venantius/ultra "0.5.2"]
                  ;; refactor-nrepl is used by clj-refctor.nvim
                  ;; 2.4.0-SNAPSHOT is required for compatibility with cider/cider-nrepl 0.17
                  [refactor-nrepl "2.4.0-SNAPSHOT"]
                  [cider/cider-nrepl "0.18.0"]
                  ;; eastwood is a linter
                  [jonase/eastwood "0.2.6"]
                  ;; ancient checks for outdated deps
                  [lein-ancient "0.6.15"]
                  [lein-voom "0.1.0-20180617_140646-g0ba7ec8"]]}}

If you suspect they might be causing problems, I can try just the cider-nrepl

Ultra might be a problem

Yes, disabling ultra makes pprint work in fireplace again

Weird that it sometimes worked, and that it works always in lein repl

Thanks for the tip!