so at first glance you might be missing that mavlink 2 does 0 padding trimming.
ie all 0’s at the end of a packet are removed.
so if you send a message that has 20 0’s at the end of the packet, they will all be removed, and transmitted without the 0’s. on reading however the V2 packet, they will be reconstructed as 0’s
in MP, turn on the console (help > console, and restart mp)
then in mp settings, tick the debug checkbox down the bottom.