To any/all MSFT personnel on here: this is not a BUG, but a VERY IMPORTANT FEATURE REQUEST. Please please please prioritize this, as it will provide a lot of real value for a lot of people who use Outlook.
The "default reminder" feature today only applies to invitations that you create yourself. It doesn't apply to invitations created by other people. Instead, those invitations will come with whatever notification, or none, that they set. You can override it yourself, but that has to be done manually, each and every time.
And sometimes we forget, and then we're late to / miss meetings. That's a real problem. And one that you can address with a pretty darn simple feature.
[ X ] Always set a reminder on incoming meeting invitations: [ 15 minutes ]
^^ A single check box, with a selection for the timing of the reminder. It should be a backend/account setting (not a local/client setting). If checked, then any invitation that comes in from another sender (regardless of whether they're in my organization or external) without a reminder set gets a reminder added as directed.
Seems pretty simple logic, but hugely valuable.