I've heard of people being denied for this reason
This issue is bigger than an AVVO answer, but you probably heard correctly. Washington does NICS checks for CHLs issued on or after July 22, 2011. If that card is registered with the government, there is a good chance that it will be reported to NICS. An attorney at the FBI NICS division told me that an applicant having a medical marijuana card is a "slam dunk" for a NICS denial. A NICS denial means that the person is prohibited from purchasing and/or possessing a firearm. Talk to a Washington firearms attorney familiar with your state's law.
This is for general informational purposes only. It does not constitute legal advice and does not establish an attorney-client relationship.