I assume that the OP means that the markers should stay locked to a specific picture frame, and in that case, even with the tempo being changed, the markers should still stay locked to that same picture frame. That is how I score movies in Logic where they are called scene markers, and that is the idea behind placing a marker with video. However I tried it just now and in Dorico the markers do indeed move to another frame when changing the tempo. So I assume that markers in Dorico have another function than in a Daw?