So from what I understand,
you try to send additional JSON data alongside "assets": "%TRIGGER_ASSETS%",
but it removes it when sending the HTTP request?
So that would mean; {"rule": "%RULESET_NAME%", "assets": "%TRIGGER_ASSETS%"} works, but {"rule": "%RULESET_NAME%", "assets": "%TRIGGER_ASSETS%", "mydata": 2} does not?
Could it be specific to your number value that is in there?
I’m happy to create an GitHub issue if this is a bug in our platform.