There are multiple Project Tasks where the Planned End date got updated and it's matching up with the planned duration.
Planned Start Date, Planned End Date and Planned Duration fields are configured in two places.
- On the top of all the sections
- Under the Dates section
If below fields are configured are in 2 places on the same form, Remove it from one either from the section or at the top of the form by navigating to Configure->Form Layout.