Why are dark stains appearing after product application?

Tannin staining can occur with any kind of wood. Most tannins are water-extractive. If the dry oil, stain or paint film is intact and discolouration occurs at a later date, then staining is caused by water within the surface. The moisture will migrate and eventually carry the tannin from within the wood to the surface of the paint film, and then the resulting stain appears.