I’m having a similar issue. This is gonna be long-winded.
On 2 bases now, I have walls just lose stability. Only walls and frames it seems and only in a vertical column. It’s only happened on 1 spot in my base. Both bases are big. 30 tiles by 60 tile and 10 tiles high at least. I play single player, no co-op and have building decay and the purge disabled. Building damage taken is set to 0.1 and NPC damage is set to 0.1. I just like building in single player so I tried to disable or minimize anything that would damage my buildings.
The first time I encountered this issue I had 1 column of walls refuse to maintain stability. I ended up putting foundation pieces in place of the walls and frames because they didn’t lose stability like the walls in this particular spot did. Not sure if this matters, but this base was built on sand and with many different styles. I’m pretty sure I used pieces from every DLC. Like I said I like to build. Anyway, I was happy and continued to build for a good week. Then one day I log in and my whole base lost stability all at once. Every single item, chest, foundation, wall, furniture, stair, you name it. All lost stability. Even had thrals in midair because they were standing on a wall when I last logged out. Event logs had over 200 entries of just this piece lost stability and that piece lost stability. Sure I had some structures that pushed the limit stability wise, but I had most of the structure at 100 stability. I chalked it up to a bug and rather than attempt to build again, I started a new game and picked a new location. One on top of a rock.
This brings me to my current issue. I have 4 walls stacked on top of each other connected to a foundation. 2 frames on top of 2 walls. Walls or frames are connected to them on each side. Yesterday I noticed the walls were missing in the exact same fashion as my previous build. Only a 1 square column affected. Surrounding walls and frames remain at 100 stability and intact. I checked the logs and these 4 walls lost stability. I rebuilt them. Repair hammer had the walls at 100 stability for all 4 walls. Since this happened before, I exited to the menu and immediately logged back in. All 4 walls were missing again and the event logs state they again lost stability. I tried connecting ceiling tiles to these walls and then to the adjoining walls. Exited to the main menu and immediately logged back in. The 4 walls were gone having lost stability but all of the ceiling tiles remained. If I replace the walls with foundation pieces, the foundation pieces stay. This part of my base is mirrored on the other side. The otherside has no issues with this section. My base is on top of flat mountain/hill area overlooking the sinkhole. It’s stable ground. Also there are no NPCs on this mountain to damage anything.
I haven’t tried to build a different style of wall and frame. I’ll try that tonight. I’m so afraid I’ll log in one day and have my whole base lose stability again. I have a save, before I built the structure that has the unstable walls, backed up to the cloud. I’m thinking about re-installing the game but I’m not sure that will fix the issue.
EDIT: All walls affected currently are Aquilonian and standing on an Aquilonian foundation which is connected to flat ground. Also all walls were built as Aquilonian. I didn’t upgrade them from another style. I read somewhere that might cause issues.
EDIT 2: This all happened in January 2020 so between the last patch and this current patch.