I've uploaded a short video here: (it says it's still "processing" but it seems to work for me)
In it, you can see me completing a city towards the south of the landscape. It includes one normal follower and one phantom (and a builder) belonging to blue (me) and a wagon belonging to green (jungleboy). I place one tile and am given a bonus turn. On my bonus turn I complete the city. I am then prompted to place the green wagon and neither player can continue until I skip this.
Hopefully this demonstrates the problem sufficiently.
PS - Although it's only minor, you can also see the problem with the chat function in this video