Keychain autofill stopped working on iPad Pro

Keychain autofill stopped working on my iPad Pro (iOS 11.3) but still works on my MacBook Pro and iPhone SE. I have had no problems with autofill on the iPad Pro since purchasing it last June, until a few weeks ago. I made no changes in settings but suddenly when accessing any web page the username and password spaces were blank. This applies to all websites that require login information - including webpages that continue to autofill perfectly well on my MacBook Pro and iPhone (so it isn't a security issue with awkward websites). I've followed the Apple Support instructions, and all the obvious settings are correct: Safari private browsing is off, AutoFill is on, Names & Passwords is on, Credit Cards is on. The App & Website Passwords still has a complete list. I have rebooted (several times), switched the various setting off and back on again etc. What else can I do?