Looks like this:
(https://cdn.discordapp.com/attachments/522836436270055435/939282476583510066/unknown.png)
Basically, the way you build last, lasts here.
Which just doesn't look right at the end of a tunnel, as the portal isn't displayed that way.
This visual bug only happens in underground mode.
In tests in never showed a portal in the away facing end. I could not generate something like you showed.