Hello pdmeloon,
If you haven't already, restart your iPhone to see if fresh connections help to get location-based reminders working again: Restart your iPhone - Apple Support
1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
In addition to restarting, make sure your software is updated as updates include bug fixes and important security updates. If an update is needed, back up your iPhone first. Both steps can be found here: Update your iPhone or iPad - Apple Support
If the behavior continues, let us know more to continue narrowing this down:
- As menu options can vary, which exact software version shows in Settings > General > About?
- If you tap Settings > Privacy > Location services, do you see "Location-Based Alerts" is turned on?
Keep us posted.
Take care!