The block mined by a Silk Touch tool with 1 durability remaining breaks as if mined by a regular tool. For instance, a coal ore mined using the final durability point of a gold pickaxe will drop coal, not coal ore.
To reproduce:
/give @p diamond_pickaxe{Damage:1560,ench:[{id:33,lvl:1}]} 1
Duplicates MC-136865.