I searched for an option that might control this, but didn't find anything. I don't have Leave selected turned on. The highlighting looks like selected text (it's highlighted with the selected text background), but it is not selected. The highlighting is only temporary -- after about 4 seconds it disappears. And if I press delete before the "highlight" goes away, only the first character of the found text is deleted instead of the entire highlighted text (as occurs if I turn Leave selected on).
Interestingly, if I turn Leave selected on, then for the first 4 seconds the found text appears with the selection background and white text. After 4 seconds it reverts to normal selected text color. With Leave selected off, it's highlighted like selected text and after 4 seconds reverts to normal text.
With VS 21, this highlighting occurs for both the first search (search-forward/backward) and repeated searches (search-again). With 22 beta1, the highlighting is only on search-again.