(Post SU2) I noticed that within the L-39 selection menu if you attempt to select a different default livery the image of the aircraft disappears and your left with a spinning icon in the lower right.
I’m however able to select my MSFS 2020 L-39 liveries that I’ve enabled. And they will show up within the L-39 selection area when selected. This is with the MS store Premium Deluxe variant of 2024.
I notice this issue with only certain aircraft in the menu.