Vl carotid bilat post TIA found:l internal carotid stenosis 50-79% by velocity. F/u with neck contrast mra, shows nothing normal study! How can it be?

Velocity accurate. Most likely accurate ultrasound. Repeat at 6 months. Unlikely a dissection with resolution- but possible. Or increased flow after tia? Would be unusual. Mra has value but some limitations of stenosis measurements.
Angio. If there is no obvious source for TIA other than carotids and with two conflicting studies of the carotids , then if it highly suspicious to be carotid in origin then you need to have conventional angiogram ( catheterization ) which is the gold standard or at least a third study with ct angiogram.