Blender Version
Broken: 2.80 (Windows 64, 8ac0fef4dfd1)
Worked: 2.79 (Windows 64), 2.80 (Windows 64, dc3b5024be1a)

Short description of error

There is no difference in the geometry that is created regardless of what the "span" value is changed to when running bpy.ops.mesh.fill_grid(). I have verified this both in scripts and the Python Console.

The span keyword used to work in Blender 2.80 the same as it did in 2.79, but the keyword appears to have stopped working after this commit was applied:

rB3d8cbb534f8277840b51f0ec7003f77ee2391f16

-            if (RNA_property_is_set(op->ptr, prop_span)) {
+            /* Only reuse on redo because these settings need to match the current selection.
+             * We never want to use them on other geometry, repeat last for eg, see: T60777. */
+            if ((op->flag & OP_IS_REPEAT) &&
+                RNA_property_is_set(op->ptr, prop_span))
+            {

Exact steps for others to reproduce the error

• Add a cylinder with 32 vertices, and set the cap fill type to nothing
• With the cylinder selected, switch to edit mode
• Make sure only the top edge ring on the cylinder is selected
• Switch to the scripting workspace / tab (so the python console is available)
• Paste the following code into the python console and execute it
• bpy.ops.mesh.fill_grid('INVOKE_DEFAULT', span=3, offset=0, use_interp_simple=False)

The bottom 2 screenshots show the differing results after running the above command in both 2.79 and 2.80

2.79 results:

2.80 results: