Long, hard lump under eyes 6 months after fat grafting - is it scar tissue or fat?

Treatment of the irregularities after fat grafting may be very difficult! It would be in your best interest to let result settle (lump may further decrease is size) for another 6 months. If issue is not resolved, second opinion consultation with a plastic surgeon with excellent reputation in fat grafting is recommended.

I generally recommend against using fat grafting for facial augmentation especially around the eyes.
What do you not present with is common.
Compared to grafting fat fillers are more precise, predictable and far more forgiving.
Anybody who injects fat should also be ready to remove fat or deal with the consequences of performing the procedure. in this area the best way to remove areas of necrotic fat/scar tissue it’s usually by direct excision.There are no surgical treatments that will work.
Follow up with your provider and ask him or her to do a revision and remove the lump.
This shouldn’t be rocket science through any experienced plastic surgeon.
Direct excision can leave a visible scar but the area tends to be very forgiving and how it heals especially if done by a skilled provider.
Your outcome is a perfect example of why I steer people towards using fillers and away from using fat transfer.
Fat transfer can be good for using diffuse overall volume. around the eyes the thickness of post tissue paper soft abruptly and this changes how fat survives. The area is unforgiving for even minor contour irregularities.
If your provider can’t help you then I recommend scheduling a few in person consultations with other local board-certified plastic surgeons who do a lot of facial fat grafting.
Best,
Mats Hagstrom MD