Upstream bug report: https://bugs.kde.org/show_bug.cgi?id=412810#c1 It seems that no font is being installed to the initrd, causing no bullet points to be shown for each character typed. This causes the input box to stay empty all the time.
As far as I'm aware this package simply installs files. It is your job as you create your initramfs to include that font. Personally I couldn't care less about plymouth and have never looked into it; wiki contributions to help the next user struggling with this very welcome.
alternatively, we could add postinst info to the ebuild; input welcome.
As said, input welcome; until then assuming local config issue.
Ah well it would be nice to have some postinst info, even if it would just link to some wiki page. I have no clue how to include said font, and I don't seem to be able to find it (although that might just be me being blind).