Arc miter bevel creates miters on straight edges I came across strange artifacts after adding a bevel modifier to a keycap model. When I set the inner miter type to 'Arc,' some vertices have a miter applied despite the adjacent edges meeting in a straight line. This seems counter to the Blender manual:

A miter is formed when two beveled edges meet at an angle.

I found it very easy to reproduce this issue without any manual modeling:

Start with a cube
Add a subdivision surface modifier set to 'Simple' and increase the subdivisions to 3
Add a bevel modifier with limit method set to 'Angle' and inner miter type set to 'Arc'

The model I originally encountered the problem on is a deformed subdivided cube. The same problem occurs regardless of whether I use modifiers or tools to create the subdivisions and bevels. I can understand why Blender might add a miter to a curved edge like the top of the keycap, but it ignored that edge. It seems to add them totally at random.
I have tested from scratch on Blender 2.82 and 2.90. Why is this happening? Is there anything I can do to mitigate it, besides applying the modifier and cleaning up?
The strange behaviour in question was a confirmed bug and has since been resolved. In the latest version of Blender (2.92.0 alpha) the issue is absent.
If you are having the same issue, update Blender.