Hello Eric,
Thanks for reaching out to the Apple Support Communities! From your message it sounds like locations you have entered are no longer present when you open up the iOS Weather app.
Based on this information check out these troubleshooting steps to begin: If an app unexpectedly quits, stops responding, or won’t open.
You mentioned that this has been happening for years. If it's happened on more than one iPhone, are you restoring your new iPhone from the backup of your previous iPhone? If you are, the next step is to back up your current iPhone data following these steps: How to back up your iPhone, iPad, and iPod touch.
Once the backup is complete, restore you iPhone to the factory settings, and set it up as new. This article will show you how: Restore your iPhone, iPad, or iPod to factory settings.
After you have set it up as new test it out to see if any locations disappear from the Weather app. If there is no change and the locations are still present, you'll want to restore the backup to your iPhone: Restore all content to iPhone from a backup.
If the issue returns after you have restored the backup, you'll know that there is something corrupt in the backup which is triggering the issue.
All the best!