SDL_ShowMessageBox failing on 64-bit Raspberry Pi

Currently SDL_ShowMessageBox() and SDL_ShowSimpleMessageBox() are not working on the 64-bit beta-test version of Raspberry Pi OS. This is using SDL 2.0.9, which is the latest version available from the Raspberry Pi repository.

Specifically, the functions immediately return -1, without displaying anything, and SDL_GetError() reports “No message system available”. This is unexpected because the functions work correctly in mainstream 32-bits Pi OS (‘Raspbian Buster’).

Can anybody shed some light on this? Is the problem likely to be in SDL or at the Raspberry Pi end? It’s important that it be fixed before the 64-bit Pi OS is formally released.

Some extra information. SDL_GetCurrentVideoDriver() is returning “x11”; that should mean the necessary functionality to support the message system is available, shouldn’t it?

Noob question, which raspberry pi is this?

It’s a Raspberry Pi 4 (the beta-test 64-bit OS requires a RPi 3 or RPi 4). Apart from the message system, every other aspect of SDL 2 that I’ve tried works correctly (including audio).