Hi there,
I hate to say it, but I think you have a type of Amaranth - unfortunately not the ornamental kind.  It's also known as pigweed and it in extremely invasive. Yours are still small and because they are in your flower beds, I recommend that you just pull them out or cut it off at the base with your hoe.
The seeds could have come to your garden from any number of sources since they are prolific seed makers, and those seeds can be viable for up to 30 years.

Below are two links if you would like to read up further.

Amaranthus palmeri (Carelessweed, Dioecious amaranth, Palmer's pigweed) | North Carolina Extension Gardener Plant Toolbox (ncsu.edu)

Palmer Amaranth: What It Is and What to Do Now | Ohioline (osu.edu)