Easy solution, delete the Activity and replace it. Something is wrong with it. 5 minutes to replace, 5 days or years to fix. Happy or Right, often exclusive states. Oracle gave Primavera a limitless database, something got tweaked in the infinite columns.
But you could... Add all Date, Constraint, Resource assignments, Relationships, Calculation and well, all columns to the View. Open a hundred activities from the project and strip out all Organization for a clean table. Look for the variant value. OR Calc the project and check the activity in the log. OR Run Claim Digger and see what it says about the Activity.OR Look at the details clicking back and forward in the Relationships Tab: Is there a 46 year Lag? (For your change with no relationships: Assign logic, including a successor and see what it does when calc'ed.) OR What is the Resource Requirement... (that is an RD of 46.26575 years)? Was there a Resource assigned, what was their Availability/Assignment/Allocation? OR Is there a Constraint or Planned Finish, so, check the dates and remove the constraint? OR Look at the Baseline. OR Is there completion date (or Planned or Constraint) for the project?