It’s very strange, I think it’s to do with screen refresh timing or something. I’ve had this problem with almost every beta right until the one before the official release of that version of iOS. Specifically, selecting/highlighting some text results in text being selected/highlighted about 3 lines up! So it becomes a bit of a guessing game, sometimes setting the text size to huge in an attempt to highlight something at the end of a chapter. I have been installing betas of iOS on my iPad Pro 9.7 and iPhone the last couple of years* but have decided to stop because of serious issues they can cause with JW Library on iPad. JW Library is working just fine on iOS 11.3 IOS 11.4 beta 1 has just been released but I won’t be trying it. It took me a long time to figure out that it was Store & Windows updates blue screening my PC repeatedly because it doesn't like that I use tiered storage. I've had a lot of problems with the Microsoft Store and read a lot of complaints about its software, trying to re-download or update apps even months later after telling it to stop downloading and uninstall them. You basically have to expect something in Windows to break when you tell it you don't want something that it came with. This sounds more like a problem with the Store or Windows itself. Microsoft has tied several core features of the Windows 10 operating system into various pre-installed software such as OneDrive and Xbox.
JW Library lists its open source licenses in the settings menu, which has a couple mentions of Microsoft. It may be possible that necessary graphics settings or video drivers for JW Library are included in the Xbox software package, but I kind of doubt it.
The only thing I see for JW Library in the Microsoft Store pertaining to Xbox is that you can install and run JW Library on an Xbox One Console.