Just for reference. I have my probe directly on the heat tape. My thermostat is set about 5 degrees higher than I want the floor temp to be. There is some heat loss between plastic/glass, so you still need to measure the floor temps to ensure that is the temp you actually want.
As for why your snake is choosing the cool side. My guess would be for security. If you are using a set up with more than one hide, you need to have identical hides, like others have said. They will sacrifice their health for feeling secure. Half log hides are terrible hides for bps, especially young ones. Pet stores don't know how to care for their animals, that's why they use those half logs. The hide needs to be small enough that the bp has to ball up and squish himself a bit inside the hide. I use plastic flower pots turned upside down with a 'doorway' cut out.