Welcome, PFLuke! In my experience, buttercream sugar syrups need to be gently stirred in the period leading up to a boil, and as it first starts boiling. Are you stirring?
Another thought, are you boiling the syrup on high? You could lower the heat as it approaches a boil to give the sugar time to dissolve. I find with all the buttercream syrups that if I lower the heat leading up to the target temperature, then I don’t have to pour it into a pyrex, I can pour straight from the pan to the eggs.
Remember, the neoclassic syrup needs to be thickly bubbling all over, not just a few bubbles in the center.
We’d love to see some pics if you’re so inclined…