To reproduce:
1. Create a (non-identical) custom background each for pre-level text and post-level text screens
2. View a pre-level text screen in-game
3. Observe that it shows the post-level text background instead
4. View a post-level text screen in-game
5. Observe that it shows the correct background
I'm pretty sure the reason this happens is that the screen is not specifically marked as pre-text or post-text as such until after the background has already been loaded, hence it always loads the post-level text background (as that corresponds to the default state if the mark isn't specifically set) even when it's being used as a pre-level text screen.
Note that this is somewhat guesswork based on symptoms described by another user, without further testing, as well as looking at the source code - I have not tested it myself so cannot confirm for sure this is the cause of the user's symptom / that this bug exists, but I'd wager this is very likely to be all spot-on.
Did some testing now. Confirmed that the bug does indeed exist, with exactly the expected symptoms. Will be fixed in 12.12.2.
Fixed in 1e962c8 (12.13) / 7799199 (12.12).