I was testing the Validate action with a string that is not displayed on my web page, but the script runs successfully. Why?
For the Validate action with a string, it is designed to search for all reply contents within this step. Which includes the object URL and Reply Content for all objects downloaded for the page. In other words, it is not matching the content displayed on the page but the raw data downloaded.
For your case, there must be an object downloaded that includes the string you used for the Validation action, and the script runs successfully. You can easily validate that by looking at the object contents in playback results from Windows Recorder.
Feel free to open a Support Ticket if you see otherwise or needs further assistance.