If I had an 8 frame box, I would always put 8 frames in it. If it was a 10 frame box, 10 frames would go in, especially in a brood box. Empty space is an open invitation to my bees to make crazy comb. I also prefer to alternate foundation and foundationless frames if I am using foundationless at all. That way, the bees have a better "comb guide".
I know that you have been beekeeping for a while. My husband and I have been beekeeping for a few decades. So I would like to let you know a few golden rules that we use for adding a box, be it a second brood box, or a honey super. Only add the new box (filled with frames) when all of the following are true:
1. Every frame has fully drawn comb, or at least 90% drawn comb - no huge gaps, and
2. Every frame is 80% full of honey, pollen or brood, and
3. Every frame is completely covered with bees.
If you add any box before those are true, the best that will happen is that the bees will not use it. The worst is that they become overrun with robbers or pests (such as wax moths, wasps/hornets, ants and small hive beetles).
So, I have some questions. I am in San Diego, CA. We generally use 2 brood boxes. It seems that you do that too in your own traditional hives. If you usually use 2 brood boxes, you should do the same with the Flow hive - only the honey extraction process is different, not the number of brood boxes. However, I have never heard of using so many fewer frames in 10-frame brood boxes, only in honey supers. Does everyone in your area do this? If you use fewer frames, you will mess up the bee space, get more drones and honey and fewer workers in the new season. Perhaps you know something that I am not understanding. Or maybe your follower boards help reduce the space, but I am not clear on the gaps between your frames. So, was your first box completely full, as I described above, before you added the Flow super? I would guess not, but only you know for sure.
The first thing I would do is inspect for queen cells. If none, add a second brood box, preferably with at least 50% foundation frames, alternating with foundationless, unless you have drawn comb. If you have queen cells, you need to consider an artificial swarm/split.
The next comment I would make is that many Flow super users have discovered that bees treat the plastic frames as "home" much more easily if you either apply melted wax, or smear some burr comb from an inspection onto the plastic frame faces. It may be that your bees feel they are short of space, because they don't recognize the smell of the Flow super as part of the hive. Hard to tell without photos or being there to inspect with you.
In summary, I would inspect for swarm cells ASAP. If found, take swarm prevention action. If not, add a second 10-frame super from Mann Lake, Brushy Mountain or your favorite Langstroth supplier (they will fit) and let them fill it. If they fill this according to the criteria in my second response in this post, add the Flow super. However, I think it would be more likely to put the super on next year, unless you have a superb nectar flow at the moment.
Hope that helps. Sorry I went on at length. Please ask if anything is not clear.