Windows Bug in CC 2018.1: First Character Ignored When Typing Over Selected Text
The recent update of InDesign CC 2018 squashed many bugs, but it looks like it also introduced a new one. Windows users report that when they select some text and then start typing, the first character they type ignored (if it is a lowercase letter).
Fortunately, a quick fix has also been identified by the InDesign team. Turning off the OpenType text frame adornment makes the problem go away. To do this, open Preferences, and choose Advanced Type, then deselect Type Contextual Controls: Show Adornment on Text Selection/Text Frame, and click OK.