But there is no actual regulation on this separation, so immigration officers must process everyone who steps up to their desk even if you were in the "wrong" queue.
Is that really the case? In Poland there are always "EU/EEA/CH" line and then there is the "All passports" line as you mentioned, and if you're Ukrainian on the land border with Ukraine and step to the EU line, you're not going through. Is that just country by country regulation?