That seems like a sledgehammer approach. If you have a lot of blogs, that is going to be DOG SLOOOOWWW. If you do go down that road, make sure you do it in the backend where it will run faster.
I would just confirm with a console.log(), just before you run the query to see what ‘stringInput’ contains. You want to be double sure that the value is making it into the function. If it is, then it’s time to call technical support, since you may have uncovered a bug. You are doing everything right from what I can see.
Here are the specs for the variable you are querying:
Plain Content (plainContent)
Description : The text of the post without any formatting.
Type : Text
Can connect to data : Yes
Can use in dynamic page URL : No
Can be sorted : No
Can be filtered : contains, hasSome, hasAll, startsWith
Read-only : Yes
It clearly says that you can use: contains(), hasSome(), hasAll(), and startsWith() as valid search criteria.
Definitely keep us posted as to what the solution was.