Updated softwear on iPhones 8 . Now I have an app that opens and quickly closes but app working fine on my iPad which also has latest update. Any thoughts?

Several things to try:

Have you restarted your device? That sometimes clears up odd issues.

If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open, read this document for suggestions of things to try --> If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support

Redownload apps and games from Apple --> Redownload apps and games from Apple - Apple Support

How to contact an app developer - How to contact an app developer - Apple Support - for problems running an app or problems restoring purchases

Find the URL for an app in the App Store to report a concern --> Find the URL for an app in the App Store to report a concern - Apple Support

If an app or feature on your device is not working or is restricted, it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution. See how to use Screen Time on an iPhone, iPad, or iPod touch by clicking here --> Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

If you still have problems click here --> Choose your country or region - Official Apple Support Select your country, then a product.