Funny, as it happens I observed this behavior for a while now, too (Xperia X with 188.8.131.52)
But I was able to attribute it to a single app: It’s ‘Olive goes shopping’ by @pawel.spoon. hadn’t had the time to report it yet.
The app uses both alphanumeric and numeric keyboard modes on one page (used to add articles to the shopping list). when exiting that very page after entering a number value with the numkeyboard active, that mode remains.
If I focus back on an input field for strings (alphanumeric) and close the app after, everything is fine.
So I suspect that there might be some QML property or other mechanism missing that resets the keyboard mode to alphanumeric since e.g. the
Sailfish Calculator doesn’t even have any text input but still it does not cause the numkeyboard to stick scratch that. Calculator does not even use any keyboard, just buttons. Sailfish People (obviously) has number only input fields.