Stress test with an EKG can show any electrical impulse abnormaility such as the LBBB. To provide some insight, electrical impulses have a pathway down the septum and then separately go into the right and left ventrical to pump (contract) and pump blood into circulation. A LBBB indicates there may be some blockage that impede the conduction of the one of the impulses that help contract the left ventricle. There can be partial blocks or there can be complete blocks, and each have a different effect on the contractility of the left ventricle. Both right and left ventricles normally contract at the same time, and a block can delay a contraction, etc. Usually not considered a serious problem, but your doctor would have further information on the subject for your specific diagnosis.
The EKG is not a very good source to determine a heart abnormaility other than a rhythm disorder. A cath evaluates the coronary vessels for any blockage, and is reliable for coronary artery disease...the EKG is not! False positive may not be the correct understanding of an EKG output...it merely has an abnormal wave form that can be caused by benign condition or artifacts. Almost always requires supporting evidence to make a dx. Subsequent tests do not comfirm so there should not be any concern.
Hope this helps and if you have any further questions or comments you are invited to respond. Take care,
Ken