I’m having a weird inconsistent fill issue when I burn. Primarily I use bamboo to engrave on. When I do smaller sections it does not seem to have any affect but when the pieces get bigger it becomes apparent .
The issue is that you’re using bamboo. Many people think that bamboo is wood. It is not, it’s a grass and it behaves differently than wood. Even on wood you get some inconsistencies of color when burning different grain types, but with bamboo it’s more difficult to predict the results.
The board is assembled from pieces laminated together and each piece burns differently. If you note, the changes in color happen at a seam in the surface. There is nothing you can do to get a consistent color in bamboo without other processing, such as paint fill or stain after the burn.