Reminders with a Repeat disappear

Some of my reminders which have a Repeat set on them (usually a custom repeat, like every 3-4 days), have started disappearing. They usually are a part of a list (whether the list is shared doesn’t make a difference). For instance, one repeating reminder that has been continuing for at least a year suddenly disappeared, and when I searched for it, the most recent one found was a completed one from October 30, with the Repeat set to Never. It seems like Apple is somehow clearing out the Repeat on these reminders, because I didn’t change the Repeat myself.