Hey, the reason is that Dungeons & Dragons 5e and Dungeons & Dragons 2024 are different RPG Systems - in your campaign you've set its system to be Dungeons & Dragons 5e so statblocks being loaded in your GM Screen are filtered to the system you've set. As you mentioned, the solution is to either use the 2020 sheets (which are for 5e) or set your campaign's system to the 2024 version so you can use its sheets in the GM screen.
Since this is just how the system is designed and not a bug, I will be closing this report