I don’t know if this is something that happens to others, but I’ve had it happen on a few sites using The Events Calendar plugin. In The Events Calendar settings, you can set two values: “Move to trash events older than” and “Permanently delete events older than”. On at least a couple sites, these values have gotten stuck. Trying to change the value and save hangs for a bit, and when the page displays again, the value has not been altered.
I contacted support and they told me to make sure it isn’t a conflict. I do not know how a setting like these could possibly be conflicted by other plugins or theme code, but I figured I’d go through the steps and report back. And darn it, disabling all other plugins, and switching to a default WordPress Year theme worked. I was intrigued and wanted to get to the bottom of what was causing the issue. I started enabling plugins in groups to narrow down the culprit. However, I once I got all plugins enabled and switched back to my custom theme, I could still change the “Move to trash events older than” and “Permanently delete events older than” settings.
So, I still don’t know why these settings got stuck, but if it happens to you, try disabling all plugins, switch to a default WordPress Year theme, change the “Move to trash events older than” and “Permanently delete events older than” settings and save. Ensure the settings were changed by reloading the Settings -> Default page. Now enable your plugins and custom theme, and try changing these settings again. Hopefully, you’ll be all fixed up and regain control over these settings.