With -Xbootclasspath/a:<path> specified the VM can't find classes in exploded modules. The problem is that <path> is prepended to the list of exploded modules:

(lldb) p ClassLoader::print_bootclasspath()
[bootclasspath= .../jdk/modules/java.base ;<path> ;.../jdk/modules/java.logging ;... ;.../jdk/modules/java.xml ;]

and ClassLoader::load_class() doesn't look past java.base:

ClassLoader::load_class(name=..., search_append_only=false, ...)

  ClassPathEntry* e = (search_append_only ? _first_append_entry : _first_entry);
  ClassPathEntry* last_e =
      (search_append_only || DumpSharedSpaces ? NULL : _first_append_entry);

    while ((e != NULL) && (e != last_e)) {
...
       e = e->next();
      ++classpath_index;
    }
  }

where:
  _first_entry == java.base
  _first_append_entry = <path>