Aha, there's a part of your problem, you're forcing nitre through into the syrup. Naughty naughty! ;-) Also I'm not too sure if cheesecloth, even several layers of it, would be sufficient to trap stuff as fine as nitre. If you really want to keep those last dregs that don't filter through, I suggest making a salvage jar, which is just that - a jar dedicated to capturing the dregs of each batch, and which you can retrieve the syrup from later, with the help of distilled H20.

FWIW when I'm bottling syrup, it goes into the largest storage jars, usually 2ls in my case. Then when someone arranges to trade goodies with me I decant from the storage jar into the 500ml or 250ml I'm sending them. By this stage any nitre has settled to the bottom and thus easy to see and ensure it doesn't get into a trading jar.

And when I do send or give jars of syrup, I ALWAYS include the following caveats:

1) That the syrup is home-bottled;
2) That the syrup may contain nitre and I then explain what nitre is, what it looks like, and that it is harmless;
3) Ditto crystals in syrup, if I am sending syrup from a batch that has shown signs of crystallizing; and
4) The jar MUST be refrigerated once it's been opened. Some years ago I sent some syrup to some friends, one of whom is a Cordon Bleu-certified chef, and someone put the open jar of syrup in the pantry...don't think it moulded, but I thought they would have known better.

As for current syrup batches, yesterday I finished the second virgin batch. Working with a batch of sap of 45l, I was expecting a little over 1l and got 2l instead! So I'm just rolling with it! And nope, this batch wasn't boosted with residue from filters, like I had done with the home second batch.