The Fae rarely kept secrets amongst themselves. Most were clever and intuitive. It was hard to keep secrets.
The draining properties of mithrl was measured important to hide.
The dwarves were prosperous in the trade.
Mithrl was a risk in the wrong hands.
Humans and dwarves would die young. A human woman became prophetically insane.
Small items had less risk.
Complete suits of armor were a classifiable risk. Fae generals knew the risks without being told. The general sought the armor ahead of battles to save the nobles. To prevent them from donning the armor.
The dwarves were superstitious by habit. They would intern fallen warriors with all their possessions.
Fae strove to continue the trade in mithrl and deplete stock piles. "For the good of all."
In fierce conflict, fae generals might win the battle and die afterwards. Sometimes along with several others.
The fae then also started that practice of the dwarves.
Burying fallen warriors along with their mithrl armor and weapons.. someplace deep and inaccessible.
Sensing that epic battles gave the armor and weapons an insatiable hunger.