Depending on the type of dehumidifier, leaving it in a small unvented
room can make it work against itself.  To dehumidify, the unit probably
has a refrigeration unit which cools coils, on which, water vapor
condenses and drops in to a storage bin.  The cooling process pulls heat
from the coolant and dumps it into the air.  The air will get warmer and
holds more water.  I don't know the exact reason for the RH spike, but
it will probably have some thing to do with the controls in the unit.
The heat generated by dehumidifiers should be vented to the outside.

Timothy Vitale
Preservation Associates
831-684-2731