What are you using as a base recipe for this? I'm assuming you don't use a custard base since you say that granulated sugar stays crystallized.
I normally use honey, brown sugar, or white sugar. With brown or white I add a small amount of corn syrup or Lyle's golden syrup to help prevent ice formation. I do use a custard base because I prefer it.
I have never tried allulose (in anything) but I don't like the taste of most of the sugar alcohols, and in baking they can give an odd mouthfeel. I haven't dug into the science, but my guess is that they bind to fats differently than other sugars.