Found a workaround for this, and it might be one for a Swift Zwift Tip @GPLama since I don’t recall knowing about it, even though it’s really obvious in hindsight. 
On the event I’ve joined which didn’t offer me a reminder I noticed that there’s a greyed out clock. Tapping on this clock brings up the reminder settings, allowing you to amend them as needed. You can change the time, add/remove from your calendar and so on. It can be done from the home screen, events list or once you’ve tapped on the event itself - i.e. showing the event description etc.
In the case of this bug, it allows you to manually add a reminder where the app didn’t ask as expected. Tested repeatedly and seems to work fine. I still wouldn’t trust the app itself for a consistent reminder but assuming it adds the event to your calendar correctly, that should be reliable.
And it’s definitely a bug, because once a reminder is set, if you repeatedly leave and rejoin the same event, the reminder options come up as expected. Go try join something else, and there’s no prompt.