Welcome @bishola92, I have neuropathy but haven't had any symptoms similar to what you have shared. While we wait for members who may have similar experiences to respond, I thought I would share this article that seems like it might be a possibility.
"Lymphangitis is an infection and inflammation of the lymph vessels. It can result from a bacterial skin infection. A person may notice red or dark streaks extending from the site of an injury, along with swelling, pain, and warmth."
--- What is lymphangitis?: https://www.medicalnewstoday.com/articles/325069.

You mentioned seeing a vascular surgeon who told you the veins are healthy. I do have lymphedema in my legs and it's worse in my right leg but it's mostly swelling and no marks or discoloration. I also had my veins in the leg checked by a vascular doctor when I was diagnosed with lymphedema but they were OK and it turned out to be caused by the lymph system in the leg which required additional tests. It might help to seek help or a second opinion at a teaching hospital or major health facility like Mayo Clinic.

Is the mark darker or larger near the groin, or does it seem to be the same size and color from the groin area to the ankle?