That interior texture is a custom layer of paint we can't seem to access using paint tools ingame. Best solution, remove blocks that have it, and paint the waynescoating back in place after replacing block. Also have noticed both window trims and wood crown have issues of unpaintable areas that...
contrary to release notes on A18, the correct folder for your modlet customicons is: Mods\YourMod\UIAtlases\ItemIconAtlas
nm, looks like it got updated to correct path. :)