Seaborne AI invasions...
I'm somewhat confused about this process.
My 'coastline' hexes are simply shallow water hexes, it could as well be deep water.
I would expect the debark hex to be a land hex (next to a shallow/deep water hex).
Apparently this does not work as you have firmly documented.

Your solution, which works at least once

Not the way I would do it in real life

It looks like there may be a game issue with this.
I'll test AI seaborne invasions a bit more and see if I can find some reliable process.
Edit:
Tried the bru-solution in Persia where each seaborne unit has its own AI-team. I also put supply on land hexes next to the debark hex since the AI is very fond of moving towards supply hexes.
Didn't work. None of the 6 units made it onto land.
I also tried my original land debark hexes, this time with supply. No cigar.
So at this point I think the AI debark process simply do not work (reliably at least).
I would love some-one proving me wrong, of course
