I also am having this issue and it is VERY ANNOYING.
iPhone 6, iOS 8.1.2
Happens almost every message I type. I've turned off iCloud Drive and it doesn't seem to have helped.
HELP PLEASE!!
It's a bug that has to do with receiving notifications while typing.
Notifications in general have been pretty clunky for a long time. Banners pre-iOS 6 used to pop up instantly. Starting with iOS 6 they're always a few seconds delayed and they now lag the keyboard as well.
Are the chopped notification sounds a part of this too? sometimes when I get 2 notification sounds back to back the 1st one gets cut off at the end.
CoreAudio in general is totally screwed up to be honest. Occasionally I stop getting notification sounds at all and I'm forced to restart.
iOS 9 wishlist: fix the bugs. Keep the features for iOS 10.
Thanks for the info.
P.S. I borrowed your signature of listing phones I've owned. hope you don't mind