missing feathers.
I agree, it sounds like a molt. I know, you read on here that the first adult molt is only going to happen at 18 months and other myths, but that is not correct. Molts can and regularly do happen at other ages.
loosing weight
Maybe not. Some hens are mostly feathers. When they molt they look like they have lost weight but all they have lost is a lot of feathers so they look smaller. When they are molting, and this sounds like it may be a heavy molt, sometimes they don't feel good. The pin feathers growing in may be prickly. They can be standoffish and just want to be alone.
the whole flock is in on it at times
Not sure how long this has been going on or what "the whole flock" looks like when this happens. To me it sounds like one or more of the seven month olds may have recently hit the maturity level to challenge for flock master. Hens can be brutal bullies when one gets knocked down or shows any weakness. Mob mentality can sometimes take over.
is about the middle size of the flock.
I personally don't think physical size really matters. Too many cases where a bantam dominates full sized chickens. It's more about the size of the fight in the chicken than the size of the chicken in the fight.
So what to do? If she is not being injured, one option is to keep doing what you are. Eventually they should grow out of it. She will probably never be the dominant hen again but one of them needs to be near the bottom in the pecking order. Once they get settled in it can be a good life. If this is a recent development they may have not sorted it out yet. So patience may be a good thing.
If you can identify one hen as the instigator you can isolate her for a while, see how the flock reacts with her gone. That is a common strategy and sometimes works. If this is recent it's possible the new dominant hen is still cementing her takeover so that adds some uncertainty to me.
You can house that hen remotely from the others, hopefully across a fence, until she finishes the molt. This may cause reintegration issues later so I don't like it but it might work. If she is actually being injured it is probably a good idea. Maybe stick a couple of her buddies in with her and see if they pick on her.
It is always possible there is something wrong with her. A flock will sometimes turn on a weak or sick member. They can attract predators and bring the entire flock to their attention so they sometimes instinctively try to remove what can be a danger to the flock. They can be brutal.
It's hard to know what is going on across the internet. It can be hard enough when you are there looking at it. My guess is that it is a rearrangement of the pecking order as your younger ones hit maturity, fairly common. But I don't know that.
Good luck!