Last block mined by silk touch pickaxe with mending is cobblestone

By using a silk touch pickaxe (in this case with efficiency 5, unbreaking 3, Mending, Silk touch and with a Haste 2 effect from a beacon) when the pickaxe breaks, the last block it mines won´t be clean stone but cobblestone. I didn´t tried this with othr silk touch pickaxes so I don´t know if it´s only linked to this specific pickaxe/enchantment and effect combination.