There are some cases or types of designs that will cause the flow to trigger a full re-synthesis of the design. These instances occur under the following conditions:
1. When changes to the top level of hierarchy are made
2. When the Synthesis settings change
3. When small designs contain few partitions
4. When more than 50% of the partitions have a change
In addition, unusually large XDC files can also trigger a re-synthesis of the full design. This will be improved in future releases.
Note: Even though it is a Synthesis setting, -mode out_of_context will not trigger a full re-synthesis.