Why are particles only covering half of a face? Applying particles to a face, they are only being applied to half the face:

This occurs both with Eevee and Cycles. It also occurs in all viewport shading modes, and a full render. This occurs regardless of the seed used. The problem is not with the weight painting - in fact, the problem does not occur in weight painting mode:

It only occurs in object mode (and the full render). The split appears to be along the 45-degree angle between the x- and y-axes. How do I fix this please? Thank you.
Thanks to @no-can-do's comment:
Switching the positions of the modifiers fixes the problem. Just place the Particle modifier above the Subdivision modifier.