Is anyone familiar with when the flower box illusion was invented,
or who created it?

To specify I'm referring to the effect where a bag is shown empty,
and multiple rectangular boxes with flowers inside are produced
from within.

I ask because I just acquired such a prop but I don't know
if its a modern idea or a less used classic.

I would say it is based on the old production purses that the German first made in the 1850. It is more of a modern idea that updated an old concept into recognizable items for today's audiences.