1) Ctrl+F, enter search text, match is highlighted in yellow. F3 highlights the next occurrence.
2) Ctrl+F, enter search text, match is highlighted in yellow. Alt+F3 removes higlighting and switches to selection highlighting.
In both cases the search panel remains open, so I would expect (2) to behave like (1) until I dismissed the search panel.
This is nitpicking, of course.
Ctrl+F remembers the last search, so the second time you open the panel, the yellow highlighting turns pale gray. Even if you retype the previous search over the old one, it won't change the match to yellow. In order to get the highlight back, you first have to delete the panel contents.
EDIT: It does seem like a bug ---it only happens with search strings of length 1.
Ctrl+I wipes the previous search, so when you retype the text, it does highlight matches correctly in subsequent searches.
This has been discussed before, I believe, but what's the difference between plain search and incremental search? Do they seem to work exactly the same due to some default settings that can be changed?