How to reproduce:
Go to Sanctum Edit, give yourself a block and a tool.
Place Block, Break Block and place the same block at the same location again.
Result: The block becomes indistructable.
Tested with: Umbris Tools, Wayfarers Totem, Decorative Ancient Wood, Refinded Sedimentary Rock
Saving and Reloading the Sanctum makes them breakable again, but since you loose your inventory each time its kinda annoying. 