What's a good syrup for home-made chocolate?

I make my own chocolate from cocoa powder, cocoa butter, and syrup.

The ingredients I was using a while ago were fine, I had success tempering and got the right kind of snap and texture. Now, using the same brand of cocoa powder and cocoa butter, I'm not getting the snap and the texture. I can't remember what syrup I was using in the past.

My method: melt the coco butter to about 55C, add equal mass of cocoa powder and stir, add an equal mass of syrup and stir. Let temperature reduce to 25C then increase to 30-31C then cool completely. I'm confident that these temperatures are reached evenly and accurately.

To reiterate, this method in the past produced a good snap. Now I get soft chocolate and when it reaches 25C it goes nearly solid whereas in the past it was fairly runny at that stage.

I'm assuming the syrup is causing problems. (The cocoa butter is from the same batch, so maybe it's degraded over the years? But I don't think that's plausible.)

I have tried off-the-shelf golden syrup, honey, and home-made invert syrup (soft brown sugar, water, lemon juice, heated to 114C). Results vary slightly but all are somewhat soft. I have tried adding the syrup in cold, warm, and hot, with no significant effects.

Do you concur that the syrup is the reason for this failing?

What's the best syrup to use?