Two things that reduce warping even when the grain is correct, because grain is necessary and not sufficient.
Counter-line everything. Whatever you glue to the outside of a board, put something comparable on the inside. A cover paper on one face and nothing on the other means one side is pulling and the other is not, and it will bow no matter how the grain runs. That is what the paste-down is doing structurally, beyond looking tidy.
Press until properly dry, not until it looks dry. Under weight, between boards, overnight at minimum, and longer for anything heavily pasted. Most of the movement happens as moisture leaves, so a cover taken out of the press after an hour has done most of its warping in front of you. Changing the blotting paper partway through helps a lot.
And the reason yours look fine and then curl days later: they were still losing moisture when you took them out. The press was holding them flat while they dried unevenly, and once released they moved.
One more: use less adhesive. A thin even coat introduces far less water than a generous one, and most beginner warping is a moisture quantity problem sitting on top of the grain problem. Thin coat, correct grain, counter-lined, pressed overnight - do all four and it stops.