btw, I saw there's some backward compatible logic when fetching config values(removing of the ":config" part), played with it a little and it seems like a bug

Interesting - that does look like a bug in the compatibility logic. I don't think we need that logic anymore (we migrated off of that earlier design well before the initial public release). Mind opening an issue to track getting this fixed in https://github.com/pulumi/pulumi/issues/new?

Done: https://github.com/pulumi/pulumi/issues/2171