Two faces linked by a single point reading as one face

Turn on ‘Hidden Geometry’ to see all the fragments of the surface you painted red. There is only one fragment that has one on the other side of the mutual endpoint that does not let you select it separately.
This is caused by the long known “bowie glitch”: (read this entire thread please)

The “bowtie glitch” has nothing to do with models being too small, that is a different “scale” story.
Your best solution would be deleting one edge (thus one fragment of the surface) and then redrawing one edge, all as shown by @paul.millet’s gif above.

To see what actually happens:

  • draw a rectangle.
  • Then drag one corner copanar over the opposite edge to create a bowtie.
  • finally try to select one face of the bowtie.
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