Historically speaking, the Dark BASIC Professional IDE (Synergy?) used to scan all of the "#include" and automatically update the IntelliSense ... but the version of it that it uses isn't actually compatible with the Modern IntelliSense., as it uses some deprecated functions; so it'll call them, but they don't do anything anymore.
At some point I'll look into it, as it should be an easy enough fix.
Just have to update it to the Modern APIs and rebuild it... but in the mean time opening the .dba should result in it updating the IntelliSense for the whole project.