In ios 11, when I used the share sheet to call up 1P to enter my password, if the 1P app was not unlocked (i.e. I hadn't entered my master password within the last hour in my case), I would be prompted to enter my master password before I could access the relevant saved password - as expected. Following this, the 1P app and the share sheet integration in Safari would remain accessible using FaceID (this is on an iPhone X) for the next hour, before locking again and requiring the master password thereafter. The point is that I was able to unlock the 1P app by entering the master password in the Safari share sheet, and the app would remain unlocked (protected via Face ID)
In ios12, if I unlock 1P using the password autofill function in Safari, the behaviour has changed. Now when I do this and then subsequently click on the 1P app, the app is locked, and I have to enter my master password a second time to unlock the app (even though I had entered my master password just before while using the safari password autofill feature).
Not sure if it matters, but under advanced - security, always show lock screen for password autofill is toggled on.