Why can I not type certain upper case accented letters using the option key with Turkish input?
Thanks to tips from @TomGewecke, I was able to find the app/shortcut key combo that took over the letters I was trying to type.
Amethyst (a great window management tool) uses Option + Shift + I to "Display current layout" and Option + Shift + S to "Select Wide layout".
I don't have any use for those functions, so I removed the corresponding shortcuts.
For future reference, I am wondering if there is a way to ask macOS for "the list of currently assigned shortcut keys and what app has defined them".