Why does safari no longer save passwords after the recent iOS 11.3 upgrade

Welcome to the Apple Support Communities!

I understand you are noticing that Safari is no longer saving passwords on your iPhone X and iPad. The resource below will provide some troubleshooting steps for this issue, including checking your settings for iCloud Keychain and Safari autofill. I've highlighted some key information here for you. Click on the link for complete details.
Frequently asked questions about iCloud Keychain​
Get help with iCloud Keychain

If Safari doesn't autofill your account names, passwords, or credit cards, or doesn't ask to save them to begin with, check the AutoFill and Private Browsing settings on your device.

On iPhone, iPad, or iPod touch with iOS 9 or later:

1. Tap Settings > Safari > AutoFill.
2. For account names and passwords, check that Names and Passwords is on. For credit card information,* check that Credit Cards is turned on.
3. Open Safari. If the navigation bar in Safari is black or dark instead of white or light gray, turn off Private Browsing. Safari doesn't save account names and passwords when Private Browsing is on.

If you can't view the passwords stored in iCloud Keychain

On your iPhone, iPad, or iPod touch with iOS 11 or later:

1. Tap Settings > Accounts & Passwords > App & Website Passwords.
2. Use Touch ID when you're prompted.
3. Tap a website to view your password.

If you don't see your iCloud Keychain items

There are several possible reasons for why you can't find your iCloud Keychain information:

• Did you restore from an iCloud backup and then notice missing iCloud Keychain items? iCloud Keychain items aren't part of iCloud Backup because they already live in the cloud. To find your iCloud Keychain items in iOS 10.3 or later, tap Settings > [your name] > iCloud > Keychain and turn on iCloud Keychain.
  
• Did you turn off iCloud Keychain on all your devices? If so, your iCloud Keychain is removed from the cloud. If you chose to delete the keychain items from your devices when you turned off iCloud Keychain, your iCloud Keychain is also removed from those devices. You need to set up iCloud Keychain and add items again. (If you turn off iCloud Keychain on all your devices and choose to delete your iCloud Keychain locally on all your devices, Apple can't recover your iCloud Keychain.)