Do agents or anyone have right to make any changes on “my behalf” (I didn’t request that) to my insurance policy?

I got letter in mail stating they did changes I requested and new bill for $1200.I called them saying I didn’t request any changes only time I called is to add kids and that was 5 months ago.

And they said it was health market place made those changes because they saw supsidy in June and none for July.

Husband own his trucking business and we are taking private insurance every year.

Can they do that and not notify prior to making any changes?