When you drop the flaps two things occur. First is that you drastically increase the camber value of the airfoil over the span with the flaps. That is going to strongly affect the negative pitching moment of the wing. If the tail is not sufficiently big enough you can get a neutral or pitch down effect instead of a nose up effect as would normally be expected by the associated shift in the effective wing incidence.

The second is that the model slows down SO quickly when you deployed the flaps that the nose up effect was over before you noticed it and the model's trim caused it to nose down to get back to flying speed. How and when you lower the flaps would determine that.

What sort of flaps you built into the model will also make a difference. Plain flaps, which look like ailerons, would typicaly cause a radical gain in lift both from the increase in camber and effective wing incidence. On the other hand a split flap, such as you'd get with a lower surface only slab of plywood added to the normal wing, will provide mostly just drag and a spoiler like effect.