Did you use any foil tape when attaching the RHP to the glass? That will help you to get the RHP to stick better with the probe sandwiched between the RHP and the glass.
Again, the problem with having the thermostat probe inside is that since you set it at 99F, that's how hot the glass is. Also, if the snake ever come in contact with the glass it will get burned. Plus, the probe can get easily moved inside the terrarium and you RHP will just keep heating up the glass.
Since you're using thr heat lamp, having the RHP isn't that important so I advise you to sandwich it between the RHP and glass or get rid of it all together.
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