This is a bug with a keyboard shortcut, similar to the collective bug 124905. Affects both the iOS app, and online on iOS. When editing a document using an external keyboard, Cmd + S saves the document. However, the next character typed isn't entered, but the previous character is deleted instead. Eg. enter "test" (without quotes), press Cmd + S, then type "a". Result is: "tes".
In a current 4.2.11 build, the Cmd+S shortcut doesn't save the document... that is another bug. On the other hand, if you type Cmd+S anyway, the first character after that works normally. So probably this bug can be resolved as INVALID now?