Execution of vertical joint in insulation and plaster between semi-detached houses

Hello everyone,
there is probably already a thread on this topic. Unfortunately, I don't seem to know the correct term.

I want to buy a newly built semi-detached house. Both halves are already finished except for some remaining work in the interior.
What I noticed on the outside: At the boundary between the two halves, there is an (estimated) 2-3 cm wide joint (expansion joint?) in the plaster & insulation. This joint is open. In the past, I have seen a few similar houses where this joint was covered with acrylic, Sikaflex, or similar.

Is this mandatory or can it also be correct depending on the execution? The houses have no basement, Poroton masonry with 16 cm insulation, and colloquially speaking "normal plaster," so not clad with facing bricks.

It is indeed an expansion joint in the building, which is structurally necessary. Closing it therefore makes sense, because otherwise pests will get in. However, you need to coordinate with the neighbor. Acrylic is out of the question, Sikaflex or equivalent is the option. If the gap is still wide, you can stuff some mineral wool in beforehand.