It would be helpful to know what kind of display interface your system is using.
I guess that is going on is this:
At boot time, the initialization of the device driver is not quite ready when plymouth starts. So it doesn’t know that you system can do graphics. Later when shutting down, the display driver is now fully functional and you see theme as you have configured it.
If you have Nvidia, someone else needs to help you with that.
I’m so sorry, I’ve forgotten to tell the display interface. It’s a laptop with a
LED HD screen and my graphics card is AMD, more specifically a Radeon HD 6480G.
I was actually having doubts about that theory, but after switching my theme to spinner and rebooting, the theme didn’t showed up as I thought it would.
Also, on shutdown in X11 sessions, the theme doesn’t show up at all. But on Wayland the theme shows up.