Yep, I have been having the same this week. Because the leaves only get caught by the swirling water round the propeller, by the time you have stopped and opened the weedhatch there is nothing to see. They don't get wrapped around the shaft or propeller, they only swirl in the water but it doesn't take much to really disturb the water flow so the boat slows down, sometimes to a dead stop.
It's very common in Autumn when the leaves start falling, but the recent storms have shaken enough loose to get it now. As I said above, when it next starts drop into neutral for a few seconds or give a short burst of reverse then carry on.