Using Outlook for iOS for personal email and calendar management
I have the same issue since the last iOS update, and it's 100% Outlook, not any other app. It occurs on all my iOS devices, one iPhone, two iPads and one mini iPad.
The app seems to lose focus and the keyboard closes. There is some background process in the app itself, probably syncing the mailbox with Exchange, which is pulling the user interface away. The problem is either a bug in Outlook for iOS or in an incompatibility between the app and the current iOS. There's no going back, so unfortunately, we have to live with a broken system until they resolve the issue.