Nevertheless, if anyone has experienced this bug and even knows a workaround, please let me know. I know this bug is hard to debug, because it is so unreliable, and based on the experiences of my other bug reports, I don't expect a fix soon. So some kind of workaround would be great.

OK; I've finally found a way to trigger this bug even with a simple project, however it can take a little longer until the bug shows up. But eventually it does show up, always when being patient enough ;-) Bug: FB15553776

I've opened a bug report, however I was not able to reproduce this bug in a "simple" project. The bug occurs in my real App, which is rather complex, and I can even reproduce it when I strip down the WKWebView in my App to the raw basic (no delegates, no observers, no script message handlers etc), so it's impossible to use the WKWebView object in any way that is not supposed to. And the fact that there's no issue in all older iOS releases indicates, that there's a new problem within WKWebView.

The question is, does originalAppVersion return the version or the build number? If it's the latter (which would be the case if the documentation is correct), then this is a problem. A new Xcode feature automatically manages the build number when uploading new builds. Xcode always starts with 1 as build number for each new version and counts upwards with new builds. So you have V1.0 B1, V1.0 B2, V1.1 B1, V2.0 B1 etc. Which means the build number is not unique and therefore is useless.