The list of calendars on an account was read once, when you connected it. Anything created afterwards — a new shared calendar at work, a project calendar someone added you to — simply did not exist as far as Keeper.sh was concerned, and the only way to make it appear was to disconnect the account and set it up again. That is a lot of ceremony for what is really a stale list.
Connected accounts are now rechecked on a schedule, so calendars that appear later are picked up on their own and calendars that were deleted stop being offered. Nothing about your existing connections changes when this happens; it is the list of what you could sync that is kept current, not the syncing itself.
When you would rather not wait for the schedule, each account has a refresh action that rechecks it immediately and tells you what it found. That is the fast path for the common case: someone shares a calendar with you, you add it in the provider, and you want it in Keeper.sh in the next few seconds rather than the next few hours.
Read more about everything Keeper.sh does.