If I got you right, then because of hen-and-egg problem, the same why you cannot use a plugin that you build with the build where you want to use it.
Did I get it right that you have from("a:b:1") for you version catalog where a:b works when published, but if it is one of the projects in the current build (include(...)) it does not work?
If so, then to build a:b you need to execute the build. But to execute the build you need a:b already built, so you have a deadlock.
If you meant a:b is coming from an included build (includeBuild(...)), then you are hitting No dependency substitution happens for Version Catalogs included as separate build · Issue #18847 · gradle/gradle · GitHub.
If your version catalog is defined in a TOML file and not via the settings DSL, you can just directly use that TOML instead of using the artifact coordinates.