You laid it on too thick, because nooks are tricky to paint for several reasons.
This was greatly exacerbated by not using primer.
Whenever the paint industry claims something, they mean it in a certain context. For instance I see a lot of brushes and rollers "for all paints". Yeah, they melt and come to pieces when I roll a marine LPU, because what they really meant was "All paints sold in Home Depot (except the garage floor epoxy)".
When they say "self-priming" what they really mean is "On drywall or pre-painted off-white surfaces" and it's a brag about their coverage.
An important job of primer on raw wood is to block moisture and oils. It might have been moisture pushing out the end grain, but probably not. Mineral-based primers made for wood do a good job of this.
You didn't mention this, but I bet the whole reason you came back for a third coat is the coverage was poor. The maple was printing through the paint, either from color, absorption or texture. This falls into two areas that are primer's job too:
- Make the entire surface consistent (behaving the same): For instance, end grain absorbs diluent and paint differently than side grains. This effect will "print through" paint, making the end grain look different. Primer's job is to stabilize this no matter what, and provide a stable base that can be sanded to remove flaws. One last primer coat is added to make the sanded work consistent. So the topcoat is covering one uniform surface that has the same texture and absorption.
- Make the entire surface the same color, reasonably near the topcoat ultimate color. Without this, any color differences in the substrate will "print through" - will be noticeable in the topcoat, because topcoats are not opaque. In particular, it is a massive fad to have all house paint be off-white. As such, architectural coatings are expecting to overpaint surfaces which are already off-white, and they don't have the strength of pigment to deal with deep colors. Primer's job is to be effectively more tint coats to cover and equalize ineqaulities, like wood grain.
The next coat of a paint can partially reactivate the last coat; that's how it bonds (without having to scuff-sand for a purely mechanical bond). I'm not sure whether your third coat reactivated the first two, or you were just hitting the third coat extra hard to avoid a fourth, but the stack ended up too thick to dry without cracking.
By splitting the "coverage" task across two totally different chemicals, you reduce that layer thickness - you are applying two thin paint systems instead of one thick one. It also helps if the primer is a different chemistry, e.g. 2-part epoxy or alkyd, and that even helps the primer do a better job.
But I would say, next time, prime with an aklyd wood primer, until the surface is monochrome and as you like. Then you will use less paint and you won't have defects arising from thickness.