From what I saw on YouTube, some people speculate that Zinnia is the Floette that AZ lost. That’s a nice theory really.
So what if I add to that?
What if Zinnia was the MOTHER of the Floette that was revived?

The Flabebe line was all females, so Zinnia having a dead daughter(revived, yes, but then she was dead before) might make sense in this way.

Since the Flabebe line’s egg flowers usually follow their mother’s(correct me if I’m wrong), it’s likely that their powers(the flower-blooming one?)was shared between the Floettes and Florges with the same type of flowers.

So here’s me guessing what happened:
When Zinnia heard that her daughter had died in war, she fled to the Hoenn region in grief, and later when AZ activated the machine, the eternal life energy passed onto her as well because of the energy-sharing, and over the years she trained to be able to transform herself into a human. The Draconids at that time accepted her(who wouldn’t take pity on a wandering woman who lost her daughter?). By the time ORAS began, she was the last one left.
Oh man, she has a badass battle theme though.