If I go about my daily life doing autofill, then the autofill pop-up asks me for my master password, and I type it in, and it unlocks fine and select the login to autofill. But the vault doesn't stay unlocked; if I try another autofill ten seconds later, it asks for my master password again in the pop-up.
This was happening on my old Galaxy S10e, and is still happening on my new Galaxy S21.
I've found I can work around this by ignoring when the pop-up asks for my master password. When that happens, I cancel autofill and go directly to the 1Password app and put in my master password. After I've unlocked the vault directly with the 1Password app, then autofill asks for my fingerprint as expected until the regular vault timeout expires. At that point, autofill asks again for my master password, and I cancel autofill to open the app, and so on.
Not the end of the world, but I don't think this is intended behavior? It would certainly be frustrating for people who didn't realize the workaround, and is still annoying to do the workaround.