It's really tough to see from the picture so I'll take a guess. It looks to me as if the center of the board may be slightly concave. If that's true, your sanding may not have sanded that part of the board to the same degree as the area around it. If you went directly from 80 grit to 220 grit that may have happened. If this were the case, the board would have adsorbed the oil at different rates across the surface. Just a guess. If you did go from 80 directly to 220 that can be problematic. You should step through the grades. I'd have done 80, 100, 150, 220. There are other grades in between these but that's what works for me.
When you sanded it back down you may have flattened that area, if it existed, so the board would be level. Raising the grain is a good idea. Hope this helps.
For what it's worth, I finish my boards by first using straight mineral oil. You can get it at Walmart to $2 a bottle. After a couple of coats of that, I use a mineral oil/beeswax combination that I make. The reason for this is that the mineral oil is absorbed into the wood while the bees wax is a top coating. To me, using bees wax as part of the initial coat can make it harder for subsequent coats of oil to penetrate the wood. I don't really know if this is true but it works for me. I also have a mixture of mineral oil/bees wax/carnauba wax to give a slightly harder wax finish coat to my cheese boards.
Very nice board by the way.