The native Debian packages are broken up into jre and jre-headless. The jre package provides the package names ‘java-runtime’, ‘java8-runtime’, ‘java7-runtime’ and so on, whereas the ‘jre-headless’ package provides ‘java-runtime-headless’, ‘java8-runtime-headless’, ‘java7-runtime-headless’, etc.
The Zulu Debian packages do not provide the ‘java-runtime-headless’ or any other variations, but there are packages in the Debian repository that depend on ‘java-runtime-headless’ or ‘java8-runtime-headless’ to indicate that they need a runtime, but not the GUI parts of it. Because the Zulu packages do not provide these package names, the package manager will not recognize them as satisfying this dependency, even though they contain a full Java runtime that would work just fine for the purpose.
Note that I am not asking for you to provide an actual headless Zulu package, as I understand that that’s the sort of thing you charge money for. But if you added the ‘java-runtime-headless’ package names to the “Provides:” metadata of your packages, they could satisfy the dependencies of packages that ask for ‘java-runtime-headless’.