How can I fix inconsistent Reminders across all devices?

I previously posted a Reminders issue that I thought was only on my Apple Watch Ultra 1 (11.3.1). I now realize that the problem occurs on all my devices: iPhone 14 Pro (18.3.2), iPad mini 6 (18.3.2), M2 MBA (15.3.2).

The problem is that Reminders flagged as completed keep showing up on all devices as if they were not completed. This applies to regular Reminders and to Medication reminders.

Both of these Reminders were marked as completed when due, but they've become Zombies which show up after they were flagged as completed.

Also, Reminder syncing is very different from device to device. Some reminders show up on some devices and others not. Zombie Reminders show up on some devices but not others.

Here's today's remaining Reminders according to my MBA:

The Meds Reminder was marked as completed when due, six hours before the screenshot. Note that Wan Phra (Buddha Day) does not appear.

And here's today's remaining Reminders according to my iPhone:

Wan Phra is the only Reminder displayed even though the 16:05 Ice reminder should be there.

And here's a "critical" Medications Followup Reminder which arrived 30 minutes after I had flagged the Medications as "taken".

I should note that I am signed in to iCloud on all devices with the same Apple ID. iCloud Reminders is enabled on all devices. This problem just started a few days ago even though I have changed nothing nor have I installed any OS update in the last couple of weeks.

Can anyone explain what's going on here and how I might fix it?