It's normal on every flight.
Basically, there is a ground/flight switch on the flight deck, weight on wheels sensors and a Power Lever Angle (PLA), basically a microswitch, which all control the position of the spoilers.
In ground mode the WoW sensors are overridden and the spoilers stay retracted. If you exceed the PLA, about 12 degrees I think, in ground mode (rare as you don't need that much power whilst taxying) the switch will move to flight mode, but the spoilers remain retracted as you are now above the required PLA.
If however, you manually select flight mode with weight on wheels and a PLA of less than 12 (as we do when we line up) the spoilers will extend. This is why you see the spoilers extended prior to take off. And, as you pointed out, they retract on power application as you've now passed the 12 degree PLA microswitch.
When you land, the switch is at flight, you will get weight on wheels AND only when you've reduced past the PLA of 12 the spoilers will extend.
A bizarre system I know but does that make sense?