The Ignore Case option in the Find dialog resets itself to the mode selected in the Project Search options pull-down after every find operation. This makes the “Find Next” keyboard shortcut entirely useless until the user leaves the Find dialog to mess with a completely different and not obviously related part of the user interface.
The standard user-interface behavior for both Mac and Windows systems is for dialog selections that override global preferences to persist as long as any other entries in the dialog. The global value should be used when opening a new instance of a dialog, but just as you wouldn’t clear the text in the Find dialog after the user performs a single Find, you shouldn’t clear the user’s selected Find options either.
Given that Scrivener persists the text as long as the project remains open, even between closings and openings of the Find dialog, it should also persist the Find options according to the same rule.
I think it would be even more useful if the Find dialog options would remain independent of the Project Search options, for example to support project-wide reviews of capitalization and punctuation, but that’s an implementation choice, not a question of right or wrong.