Looks like a manufacturing defect and shitty QC. I have worked on Ender 3s with the same problem- the vertical extrusions were not machined square on the ends and resulted in test cubes looking more like parallelograms on the Y.

Not to get pretentious- but a machinist square would be preferable for precision work.

https://www.amazon.com/Kinex-4033-02-010-Machinist-Square-0-0002/dp/B079R71LLM

The kind with uniform thickness and the circular cutout for internal measurements is what I live by.