It looks like what you are experiencing is a “random line” issue. The very short answer is it’s caused by either an open path/node or overlapping nodes. Two vectors stacked on top of each other will cancel each other out in the GUI. (Known bug.) So either the GF makes the line when it’s forced to close an open node, or two nodes on top of each other effectively trick the GUI into thinking there’s an open node and it tries to close them and you get the random line. (That’s my theory and I’m sticking to it lol.) Turning it into a raster will 100% fix it, or you can check for the above node issues. And yes, I totally agree it’s not consistent in how it occurs, it’s a huge pain in the arse and we shouldn’t have to deal with it, but it is what it is at this point.
A whole lotta other posts about it.
https://community.glowforge.com/search?q=random%20line%20order%3Alatest