If your server is responding with a 200 Ok message for pages that are
not located on your server, this could be causing the issue you are
seeing with your verification being reset. This is something I'd
advise correcting. This way when the Google crawler or other search
engine checks for a robots.txt file, they get a 200 response if the
file is found and a 404 response if it is not found. If they get a 200
response for both cases then it is ambiguous if your site has blocked
search engines or not, reducing the likelihood your site will be fully
crawled and indexed.
The other possibility that could be causing this issue is if you are
removing your verification Meta tag or HTML file after you verify.
These should be left in place on the server to enable Google to
reconfirm your verification in the future.