I've never heard of such a rule (am native), and your reasoning is fine but ther...

		makapuf 5 days ago \| parent \| context \| favorite \| on: Un Ministral, Des Ministraux I've never heard of such a rule (am native), and your reasoning is fine but there are many common examples : cheval (horse), rival, estival (adjective, "in the summer "), travail (work, same rules for -ail words)...