You have to add .defaultBlockState and COPY the block. Here an example:
public static final RegistryObject<Block> CUSTOM_STAIRS = BLOCKS.register("custom_stairs", () ->
new StairBlock(CUSTOM_PLANKS.get().defaultBlockState(), BlockBehaviour.Properties.copy(MANGROVE_PLANKS.get()).strength(0.3F, 0.3F))
Tried several varations of the path:
Writing out the full path like Luis said above
Including and excluding "textures/"
Including and excluding the file extension
Nothing seems to be working. No error for anything either. Probably the biggest clue so far.