If a job's Jenkins workspace changes then a full sync is not done.

Easy to replicate.

Create a job with a custom workspace e.g: /workspace/test/
Do a sync and build
Check in a single file
Change custom workspace to /workspace/test2
Run job.
New local workspace has only the single change.

On a jenkins workspace change a full sync needs to happen to make sure the p4 have lists and local files are actually correct