Hello jcarsley.
Thank you for using Apple Support Communities. It sounds like you're experiencing issues with your Apple Watch. Specifically, when using the Timer, it doesn't countdown properly or alert you when the timer has ended. We can understand how this would be an issue. Allow us to investigate this issue and assist in any way we can.
First, you can try some basic troubleshooting. Start by closing the Timer app on your Apple Watch. Press the Side Button on your Apple Watch, locate the Timer app and swipe to close it. Once you've done this, restart both your Apple Watch and iPhone. Then test to see if this specific issue continues to occur:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
There has been two new software updates recently released by Apple. iOS 15.0.2 and watchOS 8.0.1, if you haven't installed these updates, try updating to see if that helps you out. Just in case, the resources below can guide you through the update process:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
Update your Apple Watch - Apple Support
If you're already updated and restarting both devices doesn't help resolve your issue, try unpairing and re-pairing your Apple Watch to your iPhone:
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
Feel free to keep us updated on your results. Best of luck.