waffleberg thank you for taking the time to report this bug.
I have been trying to reproduce this issue and what I have found is that it appears that this behaviour is also somewhat dependant on the website.
Here's a comparation of orionfeedback.org and github.com on both Safari and Orion on an iPhone with a physical keyboard paired.

As you can see orionfeedback.org doesn't show any useful prefill panel, while github.com does.
I believe we are limited by Apple's APIs in this case and I am not sure there's much more we could do? In the meantime you can always tap/click on a text field (or even control + click) to open a contextual menu that allows you to pick the "Passwords" for "AutoFill".
I'm sorry I couldn't tell you we've fixed this issue completely.
Hope this helps.
Thank you for making Orion a better browser!