MultiMeshInstance3D work perfectly when built for Desktop but crash/blackscreen the app when exported for mobile. Regular MeshInstance3D work without problem.

Is this expected behavior, something that's on the ToFix list, or am I doing something wrong?
I've tested with both Compatibility and Mobile mode, on Godot 4.1.1.

It is possible you are encountering this bug.

If that seems incorrect, take a look through some of the other bugs related to MultiMeshInstance3D

If still none of those seem like your issue, please consider opening a new issue on GitHub. You're unlikely to find the help you need here on the Godot forums, but you might also try the Godot Discord server.