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Did you have a weather event like drought? Were the leaves eaten by webworms, which favor this tree? Did the leaves yellow first before dropping?

There is a serious disease called mimosa wilt, which can cause leaf and branch loss. Read the info below and if you think your tree has it, water it frequently and apply a 10-10-10 fertilizer.

https://aces.nmsu.edu/pubs/_h/H160/welcome.html

https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/2811/2811-1020/2811-1020_pdf.pdf

https://www.gardeningknowhow.com/ornamental/trees/mimosa/growing-silk-trees.htm

I suspect it's transplant shock. Keep following the watering schedule below and they hopefully will recover.

Here's an article about that: https://www.gardeningknowhow.com/ornamental/trees/mimosa/moving-mimosa-trees.htm

This article will help: https://www.gardeningknowhow.com/ornamental/trees/mimosa/moving-mimosa-trees.htm

My research has not turned up any possibility of seed pods without flowering. Is it possible it bloomed and you didn't notice? A remote possibility is it was not a seedling, but had reproduced vegetatively and bloomed before you collected it.

I suggest visiting with the local extension service and have them speak with a professor in the horticulture or botany department.

That is an Acacia! They bloom very young.

https://www.gardeningknowhow.com/ornamental/trees/acacia/acacia-tree-types.htm

Since it was just a twig, I'm wondering if it was seed grown. In that case, you wouldn't get the same properties as a Chocolate mimosa aesexually propagated.

It starts out green then turns burgundy and brown during the course of the summer. Do you have the tree planted in acidic soil? You might want to get a soil sample tested and look for pH level as well as any soil deficiencies. Especially since it isn't blooming either. Mimosa also likes moist soil, so make sure it is adequately watered.

https://www.gardeningknowhow.com/ornamental/trees/mimosa/chocolate-mimosa-trees.htm