I’ve just discovered a bug, I think! I had a good search and it doesn’t seem like anyone’s reported it before.
Start a new flow.
Select the rest (or type a note, if you insist - it makes no difference).
Invoke the dynamics popover and type a dynamic (and hit Enter).
In the bottom properties panel flick the “Beat-relative position” switch.
The dynamic disappears.
If you un-select it it will then become impossible to re-select it, both in Page View and Galley View. It’s actually still there and will affect playback.
edit: behaviour seems to be exactly the same in Dorico 1.2 and Dorico Pro 2.1.