I think the caption is wrong and the 1st pic on the 17th is 50,000
you can tell by the spray (more of it) plus on the 17th total out flow was 50,000 in the afternoon according to the data and they turned the power plant off at first until that evening......not 100% sure though
in any case, assuming a steady 40000 out of the main spillway the outflow from the power plant has been slowly decreasing over time and is now around 5000 cfs ....a few days ago it was 6800...
I suspect more debris is being washed in to the pool and they have to cut back to prevent the water backing up
Apparently the operating plan is to use the main spillway to drop the res level to just above the spillway use min level.
Then shut off flow and go excavate the river below to allow the powerplant to operate. Repeat as needed.
Quite possible, perhaps even likely, though there is a LOT less debris present this time around since the loose material has already been eroded away.
Another possibility in relation to the plant is the re-routing of the power lines. We only get to see the pictures of the linemen working on assembling the "temporary" structures, but there are also crews working in the power station at the base of the dam. It is quite possible that one or two of the original 3 lines have been taken out of service for a couple of days to allow them to re-terminate the new lines. They cannot do that work while energized. That would limit the amount of power that the plant could produce due to the lack of full line capacity.
It would make sense to coordinate that work with the operation of the spillway as much as possible.