Update: Now the union is invisible and has no triangles.
I will try to help you soon. I will edit this post with a possible answer in few hours.
Edit:
Well, I can’t do anything with broken unions. Everytime something like this happens, you should hit Ctrl + Z until the union isn’t broken, fully separated (so all Unions/Negates are back to brick form), and then ask for help.
If you close your studio session, any broken Union will completely break without any hope to revert it.
Anything union’d with csgv2 cannot be exported with as a .obj file. You have to disable csgv2, separate the parts, and re-union-- which gets very frustrating.
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Was just given this error 5 minutes after a attempted union operation where I was attempting to create a rifle’s barrel. Here is the file.
Gun Barrel.rbxm (717.0 KB)
Another error if these negated parts are separated they just disappear and don’t return to be ordinary parts.
Odd Negated Parts.rbxm (16.5 KB)
I exported a car I made yesterday with CSG as a .obj fine and opened it in Blender.
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I believe I’ve found reason as to why CSG failures occur more frequently while further away from the origin.
In my current major project, Lands of Eyzira, the map is very large (4096x stud main gameplay area). Most of my building projects for the map at the moment are at the far reaches of the map (±2048 studs X or Z, Y is still ±50 studs to 0). A very common issue I run into is tools moving parts’ positions and rotations to values like 5.999 rather than 6. While I made a thread reporting this issue some time ago (which has now been updated), it got little attention due to it being passed off simply as nothing more than float error. This issue is notable on all tools. I noted that this has an extremely negative effect to unions, even if the error is <= 0.0009 repeating (aka it won’t display in the properties menu due to the three decimal place truncation) - This is what’s responsible for most of the issues stemming from building too far out from the origin, I believe.
I thought I’d put this here since it does have importance to CSG.
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CSG v1, even when using normal bricks, creates invisible unions. The same result comes out of separating unions using CSG v1. 
While it’s clearly not optimal, if you desperately need CSG v1 for the time being (for smoothing or whatever), it still works in the Future Is Bright Demo.
(apologies if you’ve already heard about this 
)
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Yeah, I need it to work on projects that used CSG v1 (using v2 just either corrupts them or adds random faces/vertices to them). Hopefully this’ll be fixed soon.
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Make sure to report any bugs you find with CSGv2 or they can’t be fixed.
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Before I negated this part, there were no micro-displacements
After I unioned the same parts again, the position of the union parts moved
Here’s the result I got after unioning the parts together. It used to be (6.4, 5.4, 12.4).
This happens every time I try unioning or negating multiple parts. All the selected parts move around together whenever I use CSG. I cannot build anything until this bug is fixed. Help!
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Thank you for reporting this,
We haven’t been able to reproduce the problem locally, but we have an idea of what could be causing it. If anyone has a model that reproduces this issue, we’d appreciate a copy so we can address the issue and ship a fix.
Thanks
I’ve made the place available for editing. I hope this helps.
Thanks! We are looking into it.
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I couldn’t replicate it with random unioning either. Eric took me into his save file with team create and showed me his CSG poorly unioning. I tried it myself and got the same bad union. Downloaded a copy and did it offline and same bad union. This is a serious issue for us, as we have deadlines to meet for a Christmas update and CSG is a vital component.
Here is a gif of it happening:
Notice how the jeep keeps on moving down consistently when all I’m doing is negating/cancel negating the jeep. I zoom in in the end to show you it moving down live.
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Lately I’ve been noticing some of the placement values on my objects changing unexplainably, and I’m pretty sure you just pin-pointed the issue. This is a very bad bug for modelers, since it makes our lives 100 times harder, so thank you for finding and reporting it.
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Found an issue I’ve been searching for for a while (a issue that keeps re-occurring for me):
The mail slot I made can be found in this copy-able game: Bug report place - Roblox
-or-
the individual model here: Bug report part - Roblox
Hopefully it replicates itself for you guys, since I have no idea what to look for in this case.
Good luck and thanks for supporting us devs,
-2cool2beawesome