Thanks for reaching out ericwho,
It sounds like your iPhone is not giving you a low power alert so you can turn on Low Power Mode. We can help.
First, let's make sure you're using the latest software, iOS 16.5. IF you're using an earlier version, you can Update iOS on iPhone. If you're using a beta version, check out the FAQ on the Apple Beta Software Program to remove the beta software and go back to iOS 16.5.
If you continue having issues after that, you can connect with Apple directly using either Get Support or if you're in a region without direct support options, you can also Contact Apple for support and service.
Hope that helps.
Take care.