Hm that is strange.
After building the Outpost I usually build a city improvement next to it to make it connect, but since yours is in the fence it shouldn't matter.
Did you build upgrades prior to it being in the fence?
I noticed you were using Capitar... I was using Magnar, Yithril and Tarth so far.
I'll try it with Capitar myself fast.
Edit:
It worked on the very first try:

Exact way to reproduce:
1. Start a new game as Capitar. (I don't think the game options are any good but I can provide them if necessary. I also don't think the race matters)
2. Settle your first city.
3. Move Sovereign Stack whereever (doesn't matter)
4. Start training a Pioneer.
5. Move the pioneer adjecent to your city and build an outpost just like in the picture above. Also queue up the Monument just at the side of the city like shown above. Queue up the Fortify upgrade instantly after that. End turn.
6. Confirm that the outpost is integrated into your city's fence.
7. You can now queue up your 2nd fortify or whatever.
Note: Steam Game Cache is validated and -cheat is not enabled.
Please do report if you can reproduce this reliably.