Had to re-open Unity 'cause I closed everything already for the night but I just tried it:
zed -= 1000;
Fixed the issue in the screenshot.
Funnily enough 999999 also seems to have fixed it. I tested out some stuff and it vanishes at -4 and lower and at 31 and above.
(All tested in the old version.)
Now in the new version. (First of all, I get a few error messages. I made a screenshot but I assume those are because it's maybe new import and not finalized version yet.)
After the update, the number was 1000. At first it was not fixed but I restarted Unity and now it's working.
Again, I don't know how much those error mean to you but I am going to bed over here now so here they are for good measure:
Thank you for your help. I am glad to be of help!