Hi All,
Sorry about the delayed response, I’ve been flying around a bunch, and finally made it back and am catching up. 
So my guess is that the receiving service wants to get literal numbers, and not strings of numbers. The problem here is in how the custom JSON for the webhook template is defined. Since it’s defined as a JSON object, and not a string of an object, the properties themselves must be parsable JSON. Since literally {{foo}} isn’t JSON, the workaround we added was to make it a string "{{foo}}".
I had started developing a feature to provide a workaround for this, but I don’t think it was ever released. I think we’ve since had another idea that will let us fix this, but it’s something we need to implement and test, so it won’t be available for a while.
In the meantime you can publish the values as literals in your string of a json object:
snprintf(payload, sizeof(payload), "{ \"v\":%f, \"s\":%f, }", v, soc);
The webhook will still package them in a string, but you’ll be one step closer. 
I’ll ping the team to see if this is something we can get on their radar.
Thanks!
David