That's odd to hear! I certainly believe it, because I've had this issue with other password managers in the past - but I haven't come across this with Bitwarden on desktop. Mobile however, I definitely get issues where it won't prompt the autofill button on the keyboard, sometimes tapping the password field will cause it to realize its there, other times I have to use the quick settings tile to autofill.
You might be able to workaround the problematic sites though using their custom fields attributes for logins.