Is another BBL my best option to fill in the bottom flat part of my butt or are there any other options to fix it? (photos)

The location of your flat spot is predictable and often does not improve from a BBL. In fact, it can be made worse.
That part of the body can tolerate a lot of pressure.   It can tolerate your entire weight while sitting.
Pressure bearing areas of the body tend to have more connective tissue that stabilize the skin of the underlying fascia layer.
The best example is the palm of the hand. This is also an area that can tolerate lots of pressure.
If you compared to the back of the hand it’s very different.
You can pinch and tent up with the skin on the back of the hand. Now try that in your mid palm.
The skin can’t be lifted or tented up mid palm because it’s connected or anchored to the underlying fashion with these connective tissue bands.
These bands do not stretch and do not allow the area to project even when fat is added.
In the buttocks the surrounding areas have less of these connective tissue bands in those areas in contrast do expand with the addition of fat. In this sense the flat spot can look worse because the surrounding areas expand more from  a BBL.
Patients who have a flat spot on the inner lower part of their products should be told about this before the procedure so that they anticipate the flat spot will be there after in the final result.
It is predictable and can be determined before the procedure.
When this is not communicated before surgery patients often believe the surgery was not done correctly or that the surgeon did not put the fat in the right place.
Unfortunately any more fat will probably not correct the problem and can create worsened cellulite.
Cellulite is also caused by the same connective tissue bands that pull on the skin.
Patients with excessive flat spots on their lower inner buttocks may in fact not be great candidates for a BBL because of this.
With enough experience this becomes fairly consistent and a bit more obvious. Ideally this discussion should be had during an in person consultation before the procedure.
I don’t perform gluteal implants but I do believe an intramuscular implant would probably do a better job of giving projection in this area.
I’m quite certain they would be plastic surgeons are different for my opinion. My view on this simply comes from having performed this procedure over and over for the last 15 years.
Certain things become consistent and clear. Patients with this type of flat spots don’t do well with fat transfer and those with cellulite often see cellulite getting worse with the BBL procedure.
In your case your surgeon has extensive first-hand experience of a BBL. I suggest discussing your concerns with him. Since he knows firsthand how the primary procedure was performed he may have better information and may have had different experiences with outcomes as well.
A “tuck” or skin excision of the lower inner part of your buttocks to round out the shape may also help with the appearance of flatness to some degree.
Simply adding fat is probably not going to give much improvement.
Best, 
Mats Hagstrom MD