I have loads of custom session properties that have never had any issue, ranging from all the way back in the early 8.0.X versions. If you're experiencing such an issue, you should reach out to Support so that they can assist you in troubleshooting.
Interesting. All I know is that my session custom props were all gone after upgrading from version 8.1.24 to 8.1.26. I've already recreated them so it is what it is; just took a bit to realize they were gone