I've used a custom stylesheet for a couple years now, by creating a functional stylesheet .nb file and placing it in (on macOS) ~/Library/Wolfram/SystemFiles/FrontEnd/StyleSheets/myStylesheet.nb and then pointing to that file path via the option DefaultStyleDefinitions via Option Inspector (Global Preferences), so that it applies to all new notebooks by default. However, since updating to 14.3, the built-in default Wolfram stylesheet is applied instead, and if I want to use my custom one, I need to set it for every new notebook separately.

Evaluating CurrentValue[DefaultStyleDefinitions] returns the custom notebook path I set, but the notebook still uses built-in styles.

Is this a bug or am I missing something? I've tried to force the app to light/dark/system mode and restart, but the built-in stylesheet is always applied.