I saved a structure block in a converted (1.12.2->1.13) world and it worked fine. However, when I was in the negative x position, the structures I saved would not save properly. I figured that it was a save issue because I tried to load the same structures in other worlds, with the same result. All of the structures with positive x coordinates worked fine, but this may just be a coincidence. The structure name is all lowercase (it is minecraft:tdmarena2a) so that is not the problem. I tested it with a couple different worlds (one normal, one void like the world I had the bug), but could not find anything that replicated the bug. The structures that don't work have armor stands in them, but so do the other structures in the world that worked.
EDIT:
I found out what was wrong, the white outline was in a completely different location than where it was saving. See new picture. So the problem is that the outline is in an incorrect position, title updated accordingly. The show air is what the structure actually saves, and the outline way away from it.
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This bug happens when a structure block in save mode has settings for rotation and/or mirroring, which can happen when a block in load mode has those settings and is then switched into save mode without removing them. The issue appears to be that the area that will be saved is determined solely by the block's position and size settings, but the outline's position is rendered based on that plus the block's mirroring and rotation.
Thus, if you load a structure with rotation or mirroring, then switch the same block to save mode to try to save that same area, you'd actually be saving a completely different region than what you just loaded and what it looks like you're saving.
This happens in every version of Minecraft with structure blocks, including both 1.15.2 and 20w20b.
Closing as a duplicate of MC-102082, because per the previous informative comment, this issue seems to have the same cause as described in the aforementioned ticket. Thanks 🙂
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Please check if that's still an issue for you in the latest 1.15.1 stable release