There was a conflict between two repeaters that had the same fieldName (data) bound to the same controller. Example:
Controller: Events
Manages a ReportTemplate model with a custom popup Form widget that uses a grouped repeater with the field name data to define the available "fields" within a ReportTemplate
Also manages Report models through a relation controller that uses a Form widget with a regular repeater with the field name data that defines the values of the fields defined by the associated ReportTemplate.
Since both repeaters had the field name of "data", but one of them was grouped and the other wasn't, this would cause an issue in Repeater::processExistingItems() where the grouped repeater would attempt to process the ungrouped repeater's data which would then fail. This issue could easily cause many other vastly more confusing and difficult to detect issues in cases where multiple repeaters with the same field name AND the same mode (grouped vs regular) existed on the same page under different contexts. The simple solution is just to ensure that the indexInputName and groupInputName are both taking the repeater's alias into account when being generated to ensure that everything stays unique like it should.
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