Unfortunately I must remain on Catalina at the present time so upgrading is not an option for me.
Update: - I have rebooted both devices and the Mac and I do keep reminders app open on all three.
It seems, as you have previously mentioned that iCloud has its own schedule and does things when it wants which we cannot control– Correct?
I came back later and it seems that the devices are updating - just slowly.
Note: In average use most of us are probably just doing one or two things at a time so this is not a big deal.
Sometimes I am moving many items from one folder to another or deleting a bunch at once and this is where the slowness really shows up
So as long as I know that eventually iCloud will catch up, then I can live with that.
In my case, after deleting many completed Reminders - using an iOS shortcut script - this worked very well for most folders - reminders app's completed reminders were successfully removed, however a GoodTask app - still showed all the previously deleted reminders. So I remembered your previous suggestion here:
You can go into 'Settings - Advanced - Delete Local Cache' to get a refresh.
and happy to report that worked well - thanks again!