When accepting or creating meeting in Outlook Calendar (as of v2112 build 14729.20248), in certain scenarios, you may add a meeting to your calendar that does not include a reminder (whether or not you have default reminders configured). There should be a prompt to confirm you want to save the meeting without a reminder, if one is not already present.
There are two cases I often come across meetings without reminders:
1) Replying to a meeting request, sometimes it does not auto populate with a reminder for some unknown reason
2) Copy and paste a meeting or appointment may not include the reminder.