I spotted this problem sometimes on my own phone, too, however, I wasn't able to explicitly reproduce this behavior, and because no-one else seemed to have the same problem, I haven't really reported it somewhere. Now, an user reported the same problem in OTRS:
If you open Wikipedia the first time since a long time (this is just a guess, and I can't say what "long time" means), and tap on the search bar you would expect, that the search overlay "starts" and the keyboard opens. However, sometimes, the keyboard doesn't open or opens for just a moment and automatically closes again. We should investigate this and probably find a way to safely reproduce this and fix it.
Device: HTC One M9
OS: Android 6.0.1
Browser: Chrome mobile 53.0.2785.124