Reagents only test if what you are looking for is there. They don't tell you anything about what you are not specifically looking for. For example if you have both LSD and a NBOMe (a generally undesired analog), it won't tell you anything about the NBOMe. Furthermore, it is not that specific (it reacts to indoles). So it is possible for someone to mix an chemical that reacts with the Ehrlich test with, say, a NBOMe for effect and pass it off as LSD.

I don't know if it happens in the wild, but it is a possibility. By using more reagents, you may be more certain, but unless you take it to a lab with good enough equipment (such as GC/MS), you never know.