Okay, at the risk of being beaten for continually posting on this, I actually found the solution on the CC3 forums. The easiest way is to go to the advanced setting in the draw tool, and click the closed radio button to make it a closed polygon. For existing walls, right click on the explode button, choose "path to polygon", select the broken wall, and then right click and "do it". It will close the polygon for you.
I've been wrestling with this since you brought it up, because I didn't want to start mapping something knowing that there was a gap in the wall. LOL. Now that my mind is somewhat calmed, I can try to make something permanent, rather than playing with walls... This message has been brought to you by my perfectionist streak/OCD.
Here's the link to the page that discussed that fix...
ProFantasy Community Forum - Walls not completing correctly