Classic UI allowed users to add deleted tasks while modern UX is restricting it.
User receives "Error: TMA-1003: Resource assignment is either invalid or there is already an existing time entry for the assignment: Resource ID"
Release : 15.9.2
This is reproducible in 15.9.2 but not in 16.0
Our defect resolution policy is that we only patch GA and GA -1 versions.
Customer Reported Defect Resolution Policy
The resolution would be to upgrade to 16.0 with the workaround being to continue to use Classic timesheets until you can upgrade.