I posted a similar issue today.
In my case, I’m setting the process variable that I later want to filter on like:
"requestId": string(value.requestId)
The reason I need to use the string() method on this value is because it is a 19 digit numeric and if I store that in a process variable without converting it to a string, the value gets altered.
In Swagger, the values show up as "\"1234567890123456789\"" and while I can get the results in Swagger UI, I am unable to do the same using the variable search API in conjunction with REST or Operate Connectors.
My workaround has been to use the Process Instance Search, filtered on processId and sorted by startDate DESC, then use the processInstanceKey as a filter in Variable Search, then check to see if I’ve got the correct instance key or not.
I’ve tried a lot of different ways of specifying the filter but not found a solution.