Thanks for replying.
Don't ask me why but, I found that the warning messages had returned, this morning. It just needed a good night's sleep!
Although, I did notice that boot pulled in Clojure 1.9.0 - alpha, before it started working. Didn't notice whether adzerk had updated, and couldn't find evidence of it.
I should look into what my dependencies actually do for me - I guess I could've traced this through, to figure it out for myself.