i-search doesn't toggle correctly between Ctrl-S and Ctrl-R

[bdw429s]

If I hit Ctrl-S and type a word that matches a single unique line in my history and then toggle back and forth between Ctrl-R and Ctrl-S, the result disappears and is no longer matched.

Searching for some strings in Ctrl-S will not find any results, yet searching for the exact same string in Ctrl-R will find a result. Why don't both searches find the same results?

There is another behavior where I can press Ctrl-R, type a word which returns a single result, then pressing Ctrl-R again will clear the result and I can't get it back until I delete a character in my search term.