Anyway, I figured out what the problem is and it seems to be a bug with MacroDroid. If you manually adjust the default voice assist setting OUTSIDE of MacroDroid's context, then MacroDroid will not recognize this setting change. Instead, what you need to do is change the default voice assist to something else (e.g. Google Assistant). You will get a popup notification from MacroDroid that permission setting is missing. Click on the notification, it will take you to the same default voice assist setting page. Change it to MacroDroid and this time the setting will get recognized and MacroDroid will be able to toggle airplane mode on/off without issues. Alternatively, you can ignore the notification popup and just go into your macro. You should find a purple highlight warning for the airplane mode action (or whatever action that requires MacroDroid running as default voice assist). It will prompt you to change the default voice assist and just change it there.
Basically, you are taken to the same setting page, but you have to go through MacroDroid to get to that setting, rather than get to the setting OUTSIDE of the MacroDroid context.
I think the same is true for when you adjust other permission settings, such as Accessibility. I've seen this for other apps like Greenify, where if you change it outside of the app context, it doesn't recognize that the Acessibility setting is turned on.