Yes they do pull the jacket up there, it's annoying.

I do what people have said, put the brooch a bit lower in front as a counter-weight. Still, the brooch will migrate upwards unless it or the plaid is pinned to the jacket in front.

One thing I tried was to pin the bottom of the plaid up under the back of the jacket, supporting some of the weight, and making the plaid look a bit like a belted plaid. Belted plaids (I don't mean the feilidh-mor) are heavy complex garments but somehow stay in place better than fly plaids do, probably because most of the weight is supported by the cloth belt around the waist.