Making Chocolate syrup too thick

Ah, yes, that does help. Corn syrup in candy making, or in this case a chocolate syrup acts to interfere with crystals forming, as well as adding shine and helping keep things fluid. You made a very nice fudge, rather than a chocolate syrup- by forgetting the corn syrup. When you are making candy, all the ingredients usually have a part to play- and leaving one out can change everything. Corn syrup gets a bad rap, thanks to HFCS- but it's a valuable invert sugar which has not been genetically or chemically meddled with, unlike the HFCS version. So you don't need to fear it as an ingredient- though like any sugar, there's only so much of it you should eat. Chocolate syrup, though- is worth the calories!

Not knowing what recipe you used, it's hard to say. I rather doubt it was anything to do with the monk fruit you used to sweeten it. More likely you just cooked it a bit too long, which reduced the liquid total.