I have a Furnace Generator that continues to burn coal even when it's internal buffer and the 2 attached machines are full.

Running Bookshelf-1.8.9-2.2.8 and NeoTech-1.8.9-2.3.3.

Thanks

Does it just finish what it had started burning or continue to burn a new piece of coal?

I've seen the same issue.. it continues to burn coal after the buffer is full and after it finished burning the piece where the buffer was full. I didn't have a chance to do any more testing to see if it only happened in certain conditions though.