Getting {"success":false,"error-codes":["invalid-input-response"] error

It was discovered there are over 25K entries in our logs from March 21 to March 25It looks like a lot of reCAPTCHA failures with "testing@example.com" captured as the email against multiple form parameters. I do also see legitimate form submissions during this period. The failure message we are getting is mostly {"success":false,"error-codes":["invalid-input-response"]}. The thing to notice here is that after this period we did not get such spike in failures and while we are trying to submit the form the reCAPTCHA is working as expected. We are not sure how and why this issue occurred. Do we have any explanation for this?