I'm having the same experience with Mail reminders. Up until the recent beta it worked fine for me, removing the messages from the inbox and placing them in the reminders mailbox. One of the reasons I was using Spark was for the snooze function and I was hoping the Mail app would eliminate the need to use Spark. Hopefully this issue is resolved in future betas. It seems to be a work in process.
Relatedly, are these supposed to work across all platforms or just on the device where they were initiated? Do most of my email work on my Mac but would like the reminders to also show up on my iPhone. Doesn’t do much good to be reminded on a device I don’t have on/with me.