How much can wood move, how much movement in wood is normal?

Yes. That's how it is. Carpenters therefore also like to use MS Polymer as joint filler at such critical points. It is paintable like acrylic, has almost zero shrinkage, 300-500% elongation, depending on the type and price. And here we come to the point. It is more expensive compared to acrylic. 1.45 acrylic, 9.95 MS Polymer. And it is harder to work with, no wet finger.

There is now a polymer from Metylan that is easy to work with. And cracks appear without problems after weeks, then acrylic is simply hard and eventually cracks.