Why is Google indexing our robots.txt file and showing it in search results?

For some reason, Google is indexing the robots.txt file for some of our sites and showing it in search results. See screenshots below.

Our robots.txt file is not linked from anywhere on the site and contains just the following:

User-agent: *
Crawl-delay: 5

This only happens for some sites. Why is this happening and how do we stop it?



Screenshot 1: Google Search console...

Why is Google indexing our robots.txt file and showing it in search results?

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