What is the MOST likely cause of this false alarm?
The search string is not HTML-encoded.
The search string must be put in quotes.
The search string must be escaped with a backslash (\) before the forward slash (/).
The search string is not in the first 5120 bytes of the tested page.
Explanations:
The search string/htmldoes not need to be HTML-encoded for Route 53 health checks, as it is treated as a plain text string. HTML encoding is typically necessary when including special characters in HTML documents, not for search strings in health checks.
The search string does not need to be put in quotes. Route 53 accepts plain text for search strings without requiring quotes around them. Therefore, this option does not address the root cause of the health check failure.
There is no need to escape the forward slash (/) in the search string. The Route 53 health check interprets the string as a regular text search, and escaping is not required for forward slashes in this context.
The Route 53 health check only inspects the first 5120 bytes of the response body by default. If the text/htmlis located beyond this limit, the health check will fail even if the page loads successfully in a browser. Thus, this is the most likely cause of the false alarm.
I infer that the answer is:
The search string is not in the first 5120 bytes of the tested page.