Characters with power over dreams can influence the setting, but not outright negate it. To reference your example, the room would not be an open meadow with a sky that is completely separated from the character's surroundings. It would still have to exist within the event's dreamscape, but may have the desired features overlaid onto it, such as a "meadow" of flowers growing in the middle of the tunnels and sky-like images shining in through the ceiling above. These effects would be temporary and revert once the character has left the area and/or stopped focusing on it. They may also find it more difficult to alter features of the setting as they move deeper into the tunnels.

To elaborate a bit further, the dream has something of a will of its own and will push back against attempts to alter it. If a character tries to reshape the dream they'll be able to feel some of that resistance.