Page_Enhanced cache changing to .old within seconds

Page Cache has been working flawlessly for my site for the past couple of years. Thanks! However, Today, I noticed that /wp_content/cache/page_enhanced/www.site.org/* is changing to _xxx.html.old within seconds of a cache refresh. I don’t use the Garbage Collection feature. I simply remove the files in the https://www.site.org cache directory every morning via a cron job. As pages are visited throughout the day, the cache is warmed and cached versions of the files are served.

Today, however, I noticed that my response time is slower – and found that the _index.html file changes to _index.html.old within a minute. Any help would be greatly appreciated. Thank you.