So that's flat sawn with the heart side up. Moisture content is always changing in wood. The top got dried and the bottom may have picked up a little...
After looking at it again the joint isn't going to gain much added strength even with the domino's glued. I will presume the board is 3/4" thick and...
The table I am building has a 3-piece tabletop, with 2 pieces attached to a rack & pinion assembly and the 3rd piece butterflying up from underneath...