You haven't told us if you're looking it on your iPhone, iPad or Mac, so it's a little bit tricky to know where to look to reproduce the behaviour.
I have tried Safari
on Mac. If the password is too long, it will truncate it for the display. However, if you click on the Edit…
button on the top right, you can use the entire password field and confirm what is recorded in iCloud Keychain
This is a example entry I have created with fictional information, with a randomly generated sequence of 63 characters.