Wide thin boards are guaranteed to cup or warp. Even though the board is all one piece it will absorb moisture at varying rates depending on the time of the year. This is why table tops etc. are usually made up of boards not more than 4” wide. Sealing helps but on very thin boards can induce warping as the sealer may not be absorbed at the same rate.
Marty made a good suggestion about wetting the cupped side. Since you have sealed the board I would try laying a damp towel over the cupped side and use a warn iron to try and remove the warp. I would also find a way to clamp the board after ironing so that it is “over bent” as that may help compensate.
Doug