I often work with EDIFACT, but never use the VS.NET validation anymore. I have found that actually deploying the schema/pipeline in BizTalk gives me much better results, often different results as well!
In addition to the above, actually deploying it into BizTalk itself, gets you additional validation options (trailing/leading spaces for example), party validation, etc...
For some reason I never trusted VS EDIFACT validation to be honest.
Hope this helps!