If your beads are working properly they should maintain the RH. If the RH is not being maintained then one ore more of the following is true
- Coolidor is not sealing well
- Beads are not working
- Hygrometers are not correct
You have tested and calibrated the hygrometer and you have said that the coolidor seals well, so maybe the beads are not working. Are they new? You might try putting some of the beads in a ziplock with a hygrometer to verfiy that they are at 65%.
Also, (all things being equal) when the temp goes down the RH goes down -- air is less dense at lower temperatures.