As others have said, garden phlox seedlings tend to produce plants with non-descript wishy-washy pink/magenta flowers; apparently reversions to an ancestral form.

Some of the modern garden phlox are sterile.

Have always found that division is the best way to propagate hardy reliable tall old garden phlox.

Seeds in the heads?

Cutting back many phlox produces a smaller secondary bloom, nothing to turn ones nose up here in September.

Personally am also very keen on cutting down garden phlox, once bloomed, to reduce the powdery mildew inoculum in the soil which will afflict next year's phlox.

It's generous of you to want to share seeds with other gardeners and there's little reason not to other than the fact that named cultivars will likely not come true from seed. If you want to harvest the seeds, you need to wait until the seedpods ripen/turn to a tan/brown color.

As SunnyBorders mentioned, some cultivars are sterile so the seeds won't germinate but that can't be said of every type. I've grown tall phlox from traded seeds and have lots of plants growing in my various beds that I grew from seed.

The only way to be sure you share 'true' cultivars with garden friends is to divided the plants.

i dont know about your friend.. lol ...

if your seeds are falling to the surface of the ground.. and SPROUTING..

why in the world would they need to be buried ...

MOST self sowers.. do not need the seed covered.. at all .... after all.. they can not do that by themselves ....

when we plant seed.. and are told to cover such with a bare 1/16th inch of soil... that is more for moisture retention.. rather than to bury them ...

many such seed will not sprout if not on the surface ...

ken