We've just discovered that you can only have one automatic discount at a time.
As others have found, this is a pretty basic limitation with no apparent solution in the pipeline from Shopify itself.
This appears to be yet another example of Shopify deliberately limiting the functionality of their core platform offering in order to support their app/dev ecosystem.
i.e. Need that feature? Buy an App!
I understand the desire of Shopify HQ to want to foster a healthy app ecosystem.
However a basic store fee can quickly double if not triple once several apps are added to make Shopify fully functional by any reasonable (2020) definition.
Won't be holding my breath, but will be starting to look for alternatives.