Filter conditions for rules must use a : in the filter you are using to trigger the rule because we are matching a filter directly. For example:
{"filters":"size:15"} // will trigger a rule with the condition size is 15
Such rules will not match filters using = because this is a numerical filter which cannot trigger a rule:
{"filters":"size=15"} // will NOT trigger a rule with the condition size is 15
You can find more details on what types of filters can trigger rules in this documentation.