Can't reproduce this. I added code to show the barrier zone and there is no gap, at least on my machine.

It seems doesn't appears often. It also randomly appears on right side or left side of the arena gate. I tried five unmodded run and it only appears on first run. Then I tried playthrough with Godship mods which allows me go to Rigel Aurelius quickly. This gap only appears in last out of six playthrough.

(See thread forums for the images, I don't know how to add image in here)

To check if the gap bug appears in the arena, shoot a weapon between the wall and the gate. I suggest to use spreading weapon such as TK10 Trident cannon or laser cannon array, but don't use cloud weapon like PK25 Morning Star cannon because it won't works.

Here also the savegame: https://www.dropbox.com/s/crxvo8bnuqwlnte/DigaRW-1854.sav?dl=0

The game need Taipan GodShip mods from relanat, download from here:
http://xelerus.de/index.php?s=mod&id=1599&d=1

Could be round-off error. Depending on the exact location of the Arena, the segments might get placed 1 pixel off. That would account for the gap. I'll look at that.

Found the largest hole of my life when my arena opponent just left through it, here's some video of me flying through it: https://youtu.be/JSLk2UyrOyQ
Not consistent between saves, started up another save with the same mods and didn't have any wall issues.