System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits

Blender Version
Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-06-21 16:25, hash: rBb11a463e4fcd
Worked: Assert was introduced by rB819152527fd4, so any prior commit should be fine

Short description of error
Running the Make Planar Faces operator on a mesh with one non-planar face and one planar face causes an assertion failure.

Exact steps for others to reproduce the error

1. Open the attached file
2. Select either object and go into edit mode (Cube.001 was auto-generated and Plane was hand-made)
3. Select all faces
4. Run the Make Planar Faces operator
5. Assertion failure

This is the same assert that T89345 hits, but with a different operator.

Of course, this only happens on debug builds.

Relevant assert:

#ifdef DEBUG
  /* The face normal is used for projecting faces into 2D space for tessellation.
   * Invalid normals may result in invalid tessellation.
   * Either `face_normal` should be true or normals should be updated first. */
  BLI_assert(face_normal || BM_face_is_normal_valid(efa));
#endif

Stack trace: P2195