Where do you report bugs to Garmin?

You can chat or call the support centre , linked to the website. See the bottom of the page.

They will ask for device data dump etc if required as well.

support.garmin.com/.../

I tried to do that yesterday via the chat & the person told me to join the beta for the 965. I gave up at that stage as it doesn't exist yet as far as I can see but please correct me if I'm wrong.

The bug I found was a custom workout from Sunday was showing up on Monday as well on the watch & not in GC or the Android app.

Yes no beta as yet , takes a while after a new device before there is a beta for sign up.

A custom workout you scheduled? I haven't experienced that (yet anyway) . Where does it show when it shouldn't , when you start the activity or in the training calendar?

I had the workout scheduled for Sunday & completed it. Was surprised  the see it there again on the watch in the calendar widget for Monday, it didn't appear on the GC web page or on the phone app. Power cycling the watch didn't help but I think it went away when I added another customer workout for next Sunday. 

Ive had similar issue with the race calendar where deleted an event on GC app and web and it remained on the watch...eventually I did a reformat of the watch to sort it after going through a bug reporting with Garmin etc etc and no solution so decided it was irritating me enough to do a hard reset.

Hadn't considered that to be honest but I'm in Ireland so Monday morning here is also Monday in the US. 

There’s a new item added “Forerunner 965” under the beta program tab but you can’t subscribe yet in garmin connect

The calendar seems a bit shakey, it shows workouts for me although I’ve turned them off. And I can’t see other events beyond 24h in the future