There seems to be an issue with micronaut-acme (or maybe acme4j itself) that is causing the intermediate certificate to not be properly returned by the server. Most web browsers contain this intermediate certificate but OpenJDK, Graal native-images, and curl on Debian 10 can't access the server.
Adding the intermediate cert to OpenJDK's cacerts file is one workaround.
See micronaut-projects/micronaut-acme#156