Quelle chance! I think I found the answer in this thread which was originally in French:
https://askcodez.com/crystal-reports-dynamique-de-limage-en-utilisant-lurl-de-la-formule.html
Evidently, Crystal doesn’t like https. I changed the protocol to http and Voila! Dynamic QR codes!
(This may just be due to browser restrictions on our server…)