@Xicor I just tested it and I could fill on the login form of https://ownyourfuture.vanguard.com/en/home/publogin with Vivaldi, Chrome and Firefox – Android 15, BW mobile app 2025.8.1.
I only experienced this once on that form, but I think that happened because my vault was locked in between (I use a very short interval on my mobile app).
I used those URIs and settings:
Here is a new post that might also provide some ideas: [Short Guide / Tips] Autofill on Android