The latest public beta for iOS 10.3.3 appears to have a glitch that affects Multitasking on the 10.5-inch iPad Pro. Update with caution!
With every beta version of an operating system, there are going to be bugs to work out. That's why they are betas. People can report bugs so software engineers can squash them before they go out officially. It appears that iOS 10.3.3 public beta 3 has a fairly large bug that might seriously disrupt your productivity on the 10.5-inch iPad Pro. Split View and Slide Over don't seem to work properly in this version.
It came to our attention in the iMore forums that Split View multitasking isn't working for some beta testers on iOS 10.3.3 public beta 3. It seems that only built-in apps are appearing in the Slide Over panel, while third-party apps are missing entirely. Additionally, if you're using a third-party app that traditionally supports Split-View, you won't be able to trigger the feature, even with built-in apps.
This appears to be a very specific issue relating to iOS 10.3.3 public beta 3 and the 10.5-inch iPad Pro. Commenters in the forums have noted that there is no problem running Slide Over or Split View with third-party apps on the 9.7-inch iPad Pro, the iPad Air, and others. There is also no issue running the official version of iOS 10.3.2 on the 10.5-inch iPad Pro.
As a reminder, it's important to make an archived backup of your device before installing a beta so that, if you want to revert back to a previous version, you can install the archive and remove the update.
If you've come across this bug in iOS 10.3.3 public beta 3 on the 10.5-inch iPad Pro, or on any beta version using any device, please make sure to submit a bug report so Apple knows of it's existence and can squash it.