Plymouth option still allows text during boot

[jerbaroo]

Describe the bug
Enabling the Plymouth boot option boot.plymouth.enable = true; still results in text message during boot. Furthermore the Plymouth image disappears for a moment and then reappears.

https://discourse.nixos.org/t/how-can-i-set-up-silent-boot/4550/12

To Reproduce
Steps to reproduce the behavior:

1. Enable `boot.plymouth.enable = true;
2. reboot

Expected behavior
Plymouth boot logo until the displayManager password entry form is shown

Notify maintainers
@ElvishJerricco

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.67, NixOS, 20.03.3087.b4db68ff563 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.6`
 - channels(jeremy): `""`
 - channels(root): `"nixos-20.03.3087.b4db68ff563, nixos-unstable-20.09pre239318.c59ea8b8a0e"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[stale]

I marked this as stale due to inactivity. → More info

[SuperSamus]

Duplicate of #32556.