I'd guess you have HTML Encode Tokens enabled in Field Management for that field, yes?
Please provide an exact example so I can help further.
The example is as follows.
Within the json object we are sending we have a field called "firstname".
This first name sometimes includes letters such as è and à.
Now within marketo's interface there are no issues with these letters.
But once they are sent in the webhook. I check the data that was sent in the activity log of the object in marketo and instead of sending è and à, it sends \u00e8 and \u00e0 respectively
Switching to the other Form/URL encoding does not suffice as it converts these characters to another format as well.
Removing the encoding altogether does not work either as it ruins the sent booleans (instead of sending true or false it sends 1 or nothing)
I don't understand why this is a problem. Any working JSON library understands these are valid JSON characters, as mandated by the JSON spec. Perhaps they're unnecessary, but they're not broken. (And Marketo doesn't need to treat these Latin-1 characters in any special way -- though they do in the HTML context as well so there's an obvious link on the back end.)