It might be a bug. There have been a couple of threads opened lately related to this issue:
https://community.shopify.com/topic/2136102
https://community.shopify.com/post/2153962
And probably a few more if you search for “discount,” sort by date, and go back through posts over the past 2 weeks or so.
Not sure what the cause is, but I’m pretty sure you’re not the only one.