Hi Matt,
Just circling back with you on this bug. Our high priority lead dev team is scheduled to work on it tomorrow.
That said, were you able to figure out why your session data would be corrupted / modified? Seems that is the root cause of the issue as is related to your environment, that does not happen in our test environment or unit tests. I am curious if there is a plugin or theme or something that is known to corrupt the session in this way. Are you able to paste your WC diagnostic here so we can see your versions and plugins list?
Thanks,
– Sean