When using the classic installer, a video mode (vga=...) is added to the grub2 default kernel command line. When using the live installer, it isn't. If a video mode is not specified, plymouth fails to display a splash screen when using some video drivers (e.g. nvidia340).
This is fixed by the patch attached to bug 21246 (attachment 9514 [details]).
Depends on: (none) => 21246
Confirmed fixed in 6.1-rc live ISOs.
Resolution: (none) => FIXEDStatus: NEW => RESOLVED