Incorrect "Display for searches" config field behavior

Here a patch.

Thanks for reporting this issue.
However, I can’t reproduce the problem. I put two search views on the same page (one page and one block display), add a “Save search” bock. I get the warning that there are two matching searches on the page. When I select one of the displays in the Saved Searches settings (keeping it at “For all except the selected”), the warning vanishes. (Same if I do it the other way round and use “Only for the selected”.)

Moreover, I don’t really see the change in the code, except that it’s now more verbose. (Whether that’s good or bad is a different discussion.)
Can you provide the variable values for which the old and the new code behave differently?