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Mirror modifier with "Bisect" enabled produces hole(s).
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Description

System Information
Operating system: Linux
Graphics card: Intel

Blender Version
Broken: 2.91, 2.92
Worked: ???/NA

Short description of error

I have a project where a Mirror modifier with "Bisect" enabled produces a hole in a mesh:

Mirror Off:

Mirror On:

Note that the face that becomes a hole is actually a nearly-degenerate quad with a very short (but not zero-length) bottom edge, rather than a a triangle.

A similar issue seems to have been previously reported and then resolved:

https://developer.blender.org/T74172
https://developer.blender.org/rBSb900a5e4527035b7eaf02d5df4b5fcaaeee52703
https://developer.blender.org/rBb900a5e4527035b7eaf02d5df4b5fcaaeee52703

Exact steps for others to reproduce the error

Open the attached file: