When you inserted the edges for the grill vents the adjacent face was changed from four-sided to eight-sided as a result of the splitting that occurred along the shared edge.

This situation can easily occur when you are splitting polygons in a localized region of a mesh, and should be anticipated if maintaining four-sided polygons is a constraint for your polygon models.

When this occurs you’ll need to find a way to gradually divide the n-gons into four-sided regions by terminating the edge loops. Terminating edge loops is when you split a multi-sided n-gon gradually into multiple four-sided polygons.

When you do this, a single vertex may have five or more shared edges coming out of it as a result. This process lets you create regions on a mesh that have more detail while maintaining a particular mesh type—four-sided polygons in this case.

To manually split the multi-sided polygon into three four-sided polygons

1. Select Edit Mesh > Split Polygon Tool.
2. In the perspective view, click-drag on the top side edge of the top grill vent. Drag your mouse so the vertex is positioned at the bottom of the edge (see image).
   
3. Click-drag on the top edge of the multi-sided polygon and slide the vertex to the right end of the edge (see image).
4. Press the y key to split the face.

   The region above the split edge appears like a polygon but is actually four-sided because of the edges on the grill vent.

5. Split the side edge on the bottom grill vent in a similar fashion by click-dragging your mouse so the vertex is positioned at the top of the edge (see image below).
   
6. Click-drag the bottom edge of the multi-sided polygon and slide the vertex to the right end of the edge (see image).
7. Press the y key to split the face.

   Your multi-sided face is now split into three four-sided faces, maintaining the overall quad topology on the helmet mesh.

8. Press the q key to exit the Split Polygon Tool.