The desertion is a bit outragious ATM, and when traveling by land and sometimes by sea, you are forced to move away instead of getting closer, even though the route you are taking will get you there.

Hopefully desertion will be tweaked in the patch. Desertion right when you land in the target province but not enough movement to besiege. Plus once the desertion begins I don't think it stops until you besiege.

Also the AI is immune to it or at least real close to it, I saw an HRE army march from near Innsbruck all the way close to Denmark and then head back south again.

I can understand the AI being Immune to desertion but on the otherhand it's a bit too harsh on the human player at the time.