Note - I will add info after doing some more testing tonight. I did not write my map seeds or info down last night.
I ran into an issue on two multiplayer 1v1 matches last night where i was unable to build walls in certain areas. This was not an issue involving red non buildable areas. I had plenty of resources. The first match was on Confluence, the 2nd i beleive was dry arabia.
When i would select a villager to buil the wall, I would align it in green, click to build it as i would any other, and then the wall would disappear. No resources were consumed, villager did not head to location. In both instances, one end of the wall was connecting to gaia, the first time was water, the 2nd time was to trees.
I tried it multiple times at slightly different angles and was not able to do so. In the 2nd case i was able to build wooden walls in the location. I then stopped the build, tried stone again and then it worked.
The area of the map had been explored, and no troops from either player were standing in the way.
I will try to recreate this last night and pull up replays if i saved them. Please ask for any info that i can try to produce for you.