Is sucrose a hydrate? And if its soluble, why doesn’t it dissolve in water after being heated?

Sucrose is not usually a hydrate. It is hygroscopic, meaning it does absorb water from the air, but generally it does not form hydrates.
Heating sugar can cause a variety of reactions. If a chemical reaction results, the molecule is no longer sucrose, which means the solubility can be different. Also, if a large lump forms after melting, there will be less surface area, so dissolution will be slower.

Sucrose is a carbohydrate but not a true hydrate as in salt * xH2O. When heated, sucrose will decompose to form water. The residue, if heated long enough, becomes black carbon which definitely will not dissolve in water. For a true hydrate, the anhydrous residue obtained after heating will nearly always redissolve in water.