Below probably the Pro 5.5.5 changelog entry of the bug fix @anotherdave was referring to:
Bug Fix: Notification Center – Only send notifications to users with an exact role match of selected roles instead of a fuzzy match based on selected capabilities.
And the Pro 5.5.6 changelog entry of a tiny related bug fix released on the same day (Dec 4th 2018):
Bug Fix: Don’t try to get users with the selected role if no roles are selected.
Unfortunately the current Free 7.2.0 release only includes bug fixes up until Pro 5.5.2 (Oct 10th 2018). So bugs fixed in the Pro 5.5.3-5.5.8 release range are not yet available in the latest Free plugin release. It’s been 3 months since the last Free plugin was released so it will probably not be long before we can expect a new update.
Any idea why this happened and how I can prevent it in the future?
To prevent your issue from happening again in the future, make sure to select ‘Custom’ in the Hide Backend section of the Notification Center module. Then deselect the ‘All Administrators’ role and select (Administrator) users individually.
Once iThemes releases an update of the free plugin, update the plugin and then roll back the recommendation above.
Note I’m not an iThemes employee and I have not tested the above so no guarantees ??