The change in brightness is related to the exposure mode of the scene. The auto-exposure modes (including histogram-based exposure) work over multiple frames to adapt the exposure.
When the user changes the light type, the histogram of the frame changes, so in the next rendered frame the exposure will be adapted. Hovering the mouse on scene view just forces a redraw, and then you see the brightness change.
For this scenario, when auto-exposure is used, it's better to enable the "always refresh" option in the scene view. In this case you will see the brightness adapt as soon as you change the light type.