I've seen this problem pop up elsewhere and the "fix" always seems to be happenstance. Just in case I'm wrong and there is a way, figured I'd ask.
I have my Google Calendar on my personal account. With my Google account I subscribe to several calendars, such as my partner's work schedule, national holidays, other things, etc.
All of these show up correctly on my watch's Calendar Widget so long as I enable showing them in the Google Calendar app on Android. Great!
Now, my employer uses Office 365. I'm not concerned about event notifications, what I want is to integrate my work calendar with the other calendars. This was easy in terms of Google and Android - I configured Office 365 to give me an ICS link (outlook.office365.com/.../reachcalendar.ics). Adding this to Google correctly shows all my work appointments, integrates with the other calendars, and enabling this in Android has them all appear on my phone in the Google Calendar app as well. Great!
So we're attempting Outlook/Office 365 -> Google -> Android -> Garmin
But the work events don't show in the Garmin Calendar Widget. Just those from the work calendar. All the other calendars work great.
I thought this would just be a matter of enabling/disabling showing/hiding subscribed calendars in the Calendar app, but that appears to not be the case. These appointments just don't come through. I tried restarting my phone and that didn't make a difference. I disabled some of my other calendars, and those events correctly disappeared from my watch - re-enabling brought those events back, but not the work calendar events.
I'm not really sure what else I can do.
I am running a Google Pixel 3 with the latest available version of Android.