I’ve seen this at some (but not all) sites as well, and I believe what’s happening is that some websites are watching for specific input events on the password field, rather than watching for changes in its value. So when OmniWeb’s autofill logic updates the password’s value, those sites don’t notice the change unless you follow up with another input event (such as typing a space and then deleting it) that changes the value even further.
If my theory is correct, the fix would be for OmniWeb to post an input event to the password field so you don’t have to do that by hand. But I haven’t had a chance to look into that very deeply yet.
I’ve checked with a few of the web pages that have this issue, and indeed, adding and then deleting an additional space character would work in all of them. If the password field has a fixed character limit, (unsuccessfully) trying to add a space character is already enough to make it work.
In contrast, just putting the cursor in the text field (at the end of the text) and pressing Return on the keyboard does not work.
I knew I’d been able to fix the sites I was using, but I was wondering whether that also solved the problem at your bank. Thanks for confirming that it does!