Strange behaviour of the macOS progress bar

[OKonnel]

Hi! I have a 27" late 2013 iMac which, thanks to BetterDisplay, behaves exactly like an Apple Studio Display, at least with regard to definition and perfect sharpness, even when I use CTRL+the mouse wheel to zoom in on a detail full screen.
I use FileVault. So the Mac immediately asks me for my password, and then the progress bar appears. But, POORLY BEFORE I get to the Desktop, the progress bar is overlaid with another, larger and longer one, a centimetre apart. Basically, as if I had set a lower definition instead of 2560x1440.
This causes that when the desktop activating I find that the disk icons are no longer in the usual place (in the same way as when changing the definition).
How can to remedy this? Thank you 🙏