I've been attempting to get my webhook sending two pieces of data to ThingSpeak to work for a while now, but keep getting back error status 400 (which is said to be due to poor/incorrect syntax). Trouble is that I can't seem to find the cause of this tripping up.
My firmware (specifically the portion publishing):
It's unclear why the triple curly brackets worked in this case; they prevent HTML special character from being escaped, but I wouldn't have expected that to make a difference for your data.
However, at least it's working now, and triple curly brackets are almost always the right thing to use when generating JSON, so I'd go with that.