Comment # 9 on bug 804607 from Jacob W

Problem still persists. I suppose the visible severity of the problem is
determined by the size and position of the two displays.

This happens on Tumbleweed too.

I had this problem on 12.3 before updating to TW. Never got around posting a
bug report, glad someone else did. 

If I remember correctly, 12.2 also had a similar issue, but instead of plymouth
being in the wrong position, both screens were mirrored and at least one had
the wrong resolution. But it's been a long time, so I may be mistaken.

I find the following themes work for me as a workaround:
text (bad position, but at least visible for me)
spinner (actually pretty centered for me)

All other themes fail and do not show any visible password input.