That's a tricky one! I've definitely run into resource loading issues after builds, though usually not with the APK/OBB split specifically. Sounds like Unity's not quite linking things up correctly for your OBB. I've been spending a lot of time lately playing a silly physics game called Drive Mad, which ironically always loads perfectly, so I'm not much help there! Have you tried manually placing the OBB file in the Android/obb folder on your device to see if it's a packaging issue or a runtime loading problem?