Its been like that for awhile. Maybe just here in Europe.
cIOS20 merged with IOS56, right? I wonder if the IOS56 merge is causing the issue (it would be a shame if it was since its otherwise perfect). One way to test is to see what happens with the other MP3 supporting game; Endless Ocean. I've got both Endless Ocean and Excite Truck so if I get some time I'll see if i can at least replicate the problem.
Though that said wouldn't the SD support also cause to behave oddly with Smash Bros Brawl. But as LouieGeetoo said could be strange coding or other incompatability* in Excite Truck (I remember hearing Science Papa had really slow loading screens under one cIOS version).
I guess the other option is to have another cIOS installed (
Hermes cIOS) and have Excite Truck set to use that
*-Wasn't the whole point of there being so many IOS to ensure compatibility isn't lost and not having to test them.