Please tell me why this happens

That’s true. CAD drawings definitely don’t have vertical and horizontal consistency.
From now on, I’ll make sure to check before drawing.
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I’ll try creating it again. Thank you!

Thank you for all the information you gave me! I’ll try and recreate it.

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This problem can also arise if you inadvertently use the Move tool instead of the Push/Pull tool.

With nothing selected, the Move tool may grab an endpoint and change its location. This may result in the creation of diagonal edges that connect this endpoint with other endpoints that are not moved.

アドバイスをありがとうございます。

それはモデルをグループにしていても、起こりうるエラーですか?
(グーグル翻訳)

Thank you for your advice.

Is this an error that can occur even if the models are grouped?

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@miyu

Unchecking ‘Length Snapping’ helps to draw without a third snap function … e.a. to the lengths you enabled, other than the other two: to axes alignment and to already existing geometry. So you will make less mistakes in future.
Unchecking does not correct your existing geometry.

‘Color by Axes’ helps to correctly draw new geometry aligned to axes (with 'Leght Snapping off !). It does not ensure you 100% that existing badly drawn geometry is ‘On Axis’

Color by axis has a tolerance, it’s not perfect. In some cases very fine errors show as on axis when they are not. The exact tolerance for color by axis false positives is discussed in several threads here on the forum. Especially in imported plans where errors can be very small, using labels with precision set high is the most reliable method of checking square.

Length snapping is well known to be a potential source of errors in models and is really best left off. There is almost no benefit (it does not work like an invisible grid as many think it does) and it can cause very frequent errors, also discussed in many threads there.

I used to see this problem a lot back when I used the line work contained in imported DWG for modeling with. You wouldn’t think it should be a problem, but practical experience seemed to expose some vulnerability that I can’t explain. Maybe the import of floating point numbers and doing some math introduces some small error that gets exposed through later manipulation. I still draw lots of stuff in a 2D program, and import DWGs, but I mostly just use them as a tracing guide while drawing and modeling from scratch with native tools. Since I’ve been doing things this way, I haven’t had this problem in a while. One exception, though, is organic curves like contour lines. I use them straight out of the imported DWG.