On my current iPad Pro, on the first night, I had the freezing problem. However, there was a firmware update that only bumped the build number that came out a day or so before. So I restored with that latest firmware and have not had the problem since. After encountering this thread, I thought maybe it was because I enabled Hey Siri... so last night I disabled Hey Siri before putting my iPad Pro on the charger (it was at 5%) and just woke it up this morning without problems.
I do not think that Hey Siri has anything to do with the issue in my case and that the restore from the very latest firmware version is what actually fixed it. FWIW, I restored to the latest firmware then restored from backup. Some people seem to think restoring from a backup is a problem, but, I don't think it's relevant.
The version of firmware I have is 9.1 (13B144) whereas the iPad Pro shipped with 9.1 (13B143).