When you assign a flow to a layout, Dorico automatically puts that flow into the Master Frame Chain. If you happen to use that flow within a Layout (LA) Frame, Dorico still assumes that you want that flow to also appear in the Master Frame Chain. You can tell Dorico not to do this by opening any of the pages that feature a regular (typically MA) Music Frame, such as the Default or First Master Pages, in the Master Page Editor. Then use the Flow Filter at the top of the Music Frame to exclude the relevant flows. Apply and Close.