EXDATE not excluding dates in only-date recurring fields

Maybe the time should be set to midnight (first second) of the day in \Drupal\date_recur\DateRecurOccurrences::massageDateValueForStorage.

I don't recall why it is midday.

This is a little old issue, Neither recall I the problem very well, but I can tell you that I've used middays in non-time dates to make less probable a day change due to a final local timezone applied. Maybe that was also your thought...