I think you'll find that even though the password field is not highlighted by default (on some setups), it'll still work as you expect (assuming you've only got a single user setup on the box; it works too for multiple users but I'd not advise using it there). Try it.
In comment later you asked if it is a bug.
If it is, it's very likely https://bugs.launchpad.net/ubuntu/+source/sddm/+bug/1885530
(Note: I tend to ignore it, and don't re-test for it often if at all.. I can see I listed only for focal (20.04), but also was reported in groovy in QA-testing.. )
If you believe that describes your issue, and you have a launchpad account, I'd recommend you clicking the "affects me too" & leaving a comment with some details (ideally number of screens, video card & layout of monitors, ie. 2xlandscape, landscape+portrait, 2xportrait etc). That may give it some weight & may result in some attention.
If you believe it's a different issue, you can file your own bug report, see https://phab.lubuntu.me/w/bugs/ (or the generic Ubuntu bug reporting link).
Please note, I QA-tested the issue and could not see how it could be maliciously used by others (in both focal & groovy), thus my lack of concern over it... also the "affects only 1 person" I consider proof it doesn't warrant much attention (including why I haven't re-tested for it on either hirsute & impish though I'm convinced it still occurs on those too!).