If it didn't say that the elemental matter fuses to the ground and just said 'rooted in place as though anchored to an immobile object' there might be room to say that it magically anchors a target in place 'as though...'
Since the description is that the elemental matter, which is a physical encasement, is fused to something, and since that matter has hardness, hit points, etc. it is a physical object, which means it exists and must be attached to both the target and what it's fused to.
I also went and looked up the full description of the ability. It clearly says that the matter surrounds your foe in elemental matter. It's basically making a cocoon or ball or hindering encasement (as opposed to saying that crackling energy reaches from the earth and holds the target fast). It doesn't say hitting them again does anything else, like create 10-foot tendrils that latch on. It says the entangling energy fuses to the ground, (this is an assumption that most targets are affected by gravity and will be on a solid surface, this is not uncommon when abilities for PCs are written. There are basic assumptions in mind, like that the PCs are humanoid in shape, etc. It also doesn't have to go into every use eventually; What if you're underwater? What if you're floating through a void in a plane without a ground? What if direction is subjective?)
This means if you hit them again with your infusion, that ball, or cocoon, or blob, or whatever fuses to the ground (or whatever they're in contact with or close to, if they were hovering 6 inches off the floor for instance), not' creates strands that stretch across the sky' or 'fires off anchors into the rocky soil' or 'grows a sapling-like stalagmite between the creature and the soil'.
If you want to say that a creature is flying 100 feet off the ground and gets 'tethered' then a ropy stand of elemental matter shoots downward to the ground 100 feet below, go ahead. You could say it isn't ropy, maybe it's a long pillar of rigid, sap-like goo.
Just realize that now you have to keep track of that, and that theoretically anyone can cut it loose at any point along a 100-foot long dimension, or even use it to climb like a beanstalk up to the entangled creature and possibly attack it (there's no apparent qualities on how sticky or hard to climb the entangling matter is nor why the interacting creature wouldn't find it hindering or possibly get stuck to it as well). Hey, that's magic and if that's how you think the rules say it works, go ahead. I don't think the power is intended to work that way.
"We can't scale this 30 foot wall? You send your flying familiar to the top and I'll hit it with entangling infusion and we'll just climb the resulting elemental strand that shoots down like we're climbing a rope and wall. DC 5, yeah!"
The answer to your question: 'What happens to a flying creature that gets hit again (when it isn't on the ground)?' is 'Nothing'.
