When a new resource plan is added for a specific amount of hours for a given duration of weeks, the total hours and allocations are being updated to incorrect values
Most probable cause is customization of Script Includes: "RequestedAllocation" and "ResourceAllocation".
Either revert "RequestedAllocation" and "ResourceAllocation" Script Includes to Out of the Box version or revisit the logic used.
To revert to Out of the Box version:
1. Navigate to System Definition > Script includes
2. In the "Name" column search for "RequestedAllocation" and go to the record
3. In the versions related list, open the record with the source of "System Upgrades: glide-xxxxx....."
4. Go to the related links section and click on the "revert to this version" link.
5. Once prompted click ok.
Repeat the above for "ResourceAllocation"