[Warhammer 40K] How are there chapters of unknown origin, given the practice of sending geneseed tithes to the Adeptus Mechanicus; also, what happens if a loyal chapter is found to be descended from a traitor legion?

There are many Space Marine chapters whose origin is officially "unknown," even discounting the Blood Ravens and the Storm Wardens, which were left up in the air to make things more intriguing for players of the respective games. However, this is logically inconsistent with the lore. In the fluff, all chapters are required to send geneseed samples ("tithes") to the Adeptus Mechanicus to screen for mutations, ensure purity, and keep as a reserve stock for founding future chapters.

Geneseed is a highly controlled, deeply-analyzed substance, arguably one of the Imperium's most valuable strategic resources...do the Adeptus Mechanicus simply lack the ability to compare geneseed from chapter X to known primarch geneseed lines? It seems to me that the origin of all chapters would be confirmed with every single geneseed tithe.

If this is the case, what would happen to a chapter that is discovered to be descended from a traitor primarch? Are they declared excommunicatus traitoris/hereticus and purged by the Inquisition?

I did a quick search, and didn't see this question anywhere...my apologies if I missed something obvious.