This only happens when more than one vSphere Cloud is configured on the system. If a vSphere Build Step in a job is configured to use a cloud other than the first (default) one and the job is saved - the correct vSphere Cloud is saved with the job. Running the job will show that the correct vSphere Cloud was used.

Configuring the job and going to the vSphere Build Step will show that the vSphere Cloud is now reset to the first (default) one. Saving the job will set this new Cloud. If you cancel from the configuration, the original Cloud value is still saved.