Reminders app
The Reminders app will not stay upgraded on iPad and iMac with latest software installed. On my iPad it will upgrade temporarily, briefly show the shared reminders lists that have been shared to it, then they disappear and the Upgrade button turns blue and may be depressed again to Upgrade. Rinse and repeat. Same for my MacBook Pro. All software is upgraded. Reminders app deleted and reinstalled several times. Devices powered off and restarted.
iPad (10th gen) WiFi, Cellular