In 2 games, both PBEM, I have assaulted Island 10 by moving a fleet of ironclads and transports with about 10-15K men on board. I stop in the river section that runs alongside Island 10, disembark with my units by moving them to Island 10 which are active and in assault stance, next turn my river fleet is of course shot up but my assaulting force is moved to the nearest Union region with full cohesion, not a shot fired and not a man lost. Last time it was Sherman and he got magically transported all the way back to Paducah which actually saved it as Grant was assualting south and an undermanned CSA force ran right into the now strong Federal force that transported from their assault down river.
It has happened to me on Fort Donnalson & Henry as well with the exact same thing happening. I had to take both by marching my men across the river. From my 3 experiences with this "bug" it has only happened in situations where there were fort batterys involved and limited landing room on the map. I can assault larger, river adjacent regions all day with great success, not sure what the problem is with doing it above.