Stone Arch Block Breaking Randomly

[Myftix]

I've been working on a build for a while. Part of the design uses stone arch blocks. Until today, i've had no issues, but when placing this piece along a well-supported wall facing, some of the stone arch pieces immediately break or appear unstable. I've no clue why; this same pattern is holding up just fine along other walls throughout the build.
I've browsed through dozens of threads suggesting improper ground support or bugs and I'm positive that ground support is not the issue here.

if there's a way to fix this, please let me know.

Here's an image of the problematic piece in an unstable state (all surrounding blocks are a stable green): https://i.imgur.com/65Wwxkg.jpeg
This particular spot either immediately breaks the piece or shows it's super unstable.

[Myftix]

I've finagled an indefinite fix for the troublesome pieces by slightly adjusting/rotating them using the Perfect Placement mod. Still, I would like to better understand the cause of this issue, so feel free to share any helpful info.

[Mharr]

Yeah, that piece is a bugger. AFAICT it *needs* the square face to be perfectly connected to another more stable block, and that's direct face to face contact, if it's embedded at all it won't register. What you're trying to do here is have a bunch of unstable blocks all hold each other up, the curved side against the wall does nothing.

[Myftix]

Originally posted by Mharr:

What you're trying to do here is have a bunch of unstable blocks all hold each other up, the curved side against the wall does nothing.

All the other pieces are stable. just that one arch stone block is being difficult. I'll test what you said and see if I can manipulate the arch stone with a cube or something to achieve the same effect without the damn thing breaking. Appreciate the feedback.

EDIT: Using small blocks to fit the arch stone pieces worked nicely, but i think it was unnecessary. I managed to keep the arch stone blocks in a stable state by quickly placing them face-to-face before they had time to break. I turned off the "free placement rotation" feature in the Perfect Placement mod to let me snap the pieces together directly rather than use the mod to shift each piece into place at a time.
Seems like I may have been over-complicating the building process for myself lol.

[Mharr]

Not being on the snap grid definitely doesn't help, especially with stone. They don't get *zero* stability from side connections but it's close enough that it might as well be. Stone is hard to learn because it is actually perfect cuboids for alignment purposes, but then the game adds a random visual skew over that so the corners aren't 'really' where they appear to be. Setting up a run of pieces that all support each other where none of them would stay up individually is definitely valid but the build process can be very frustrating!