I was getting this same issue and I think it just means there is a schema mismatch between what you're feeding your response action and what it's expecting. Here was my solution.
Check your schema definition in your Response action. Make sure:
1) All the data types match the results, no strings being forced into integers, etc
2) All key-value pairs HAVE a data type
I built my flow using dummy data, and used that same dummy data to generate my response schema, all worked great. Then I had to add one or two additional key value pairs to the response. I used a real query result from my data source to re-generate the schema and thought I was good. PowerAutomate was happy to save.
The issue was when I added two more keys, I used real query results to generate the schema and my real results had two NULLs in them. The schema left those data types out, because it didn't know how to interpret them. Once I added "type": "string" for each of them, my issues went away.
I tried replacing the trigger, using PowerApps v2 and all kind of things. I just missed something on my schema.