[Bug] Shield can be built inside creeper in some circumstances

[shrewd yarrow]

To reproduce, create a situation where the only thing separating you and a pool of creeper is a 1 tile thin land wall.
Place a 3x3 shield construction marker inside the land, so that the 3 top pixels are marking creeper, the 3 middle pixels are marking land, and the 3 bottom pixels are marking empty spots.
The expected result is that only the empty spots become shield and the rest is inaccessible to the lathe.
What actually seems to happen is that the lathes are firing at the bottom-left pixel, and after finishing construction, an L shape of land (except the land tile) has become shield (green marker in screenshot 2). Because shield has now appeared directly over creeper, the creeper in that single tile will begin to flow out of the chamber (screenshot 3)

[whole turret]

You know, I just noticed that the lathe relies on line of sight to restrict building shields. But, I sped up shield building by making it deposit more than one pixel's worth at a time. So that probably explains this effect. I have just changed it (for the next build) to check each cell for creeper before shield is deposited.

[shrewd yarrow]

cool, i will get back to you to see if i notice a difference in the next build, but i'm sure that check likely solves it completely

EDIT: confirmed resolved in 14.4

[tulip yarrow]

We can build shields which mission? I don't recall seeing them

[shrewd yarrow]

any mission, hold tab to paint shield with terraforming tool
its explained in the video tutorials