Sorry if the title isn't that clear but it's kind of hard to summarize in one sentence. 

The issue: 

I'm on the new world and orange is trying to make landfall. 

For some reason it really wants to land in a particular area. 

We were on a non aggression pact and open borders treaty at the time.

What follows is this: 

(1st screenshot) Turn 74: I place 3 units to prevent it from landing.

(2nd screenshot) Turn 75: it moves 2 tiles and I move my units to block it again. 

Then we're caught in an infinite loop where we go back and forth between those two positions. 

It did try to move further to the right after like 5 turns but then the loop started again. 

Much later it finally sent another unit in that area ... only to stack with the 4 already there and the loop continued. :)

on a side note: I did also notice the same issue during a war where the AI was trying to land on one of my islands with a lot of units but it never unstacked them and I was able to prevent it from landing with only a couple of units. (I'll admit that, that time, I was pretty happy and relieved to be able to do that in order to survive a war against a much stronger opponent. It didn't feel as much as an exploit then :) )

What it could have done:

1. send more units to find a breach
2. split the stack of 4 and send them in two different directions to find a breach.  
3. build some ships to scout for another location
4. try to share maps to find another location
5. cross over to the left where it would have been able to reach some coastal waters. Although, I'm not sure the AI could actually see those coastal tiles, if it couldn't then it could have at least tried to move some units across the ocean tiles to see if another path was available.
6. Get "lucky" (by that I mean being automatically given) and get an event that granted swift on those units?  

Another strange issue is that, as you'll be able to see in the save file, orange was pretty much boxed in on our starting continent. Yet it had access to 2 islands on the right, one with a luxury resource and one island "south" with an unknown strategic resource yet, it never made any attempt to settle them. They were not that valuable but desperate times should have called for desperate measures I suppose. 

Achaemenid Persians Turn 75 ai blocked.ctr

Achaemenid Persians Turn 81 ai try something new finally.ctr