The FP is not a factor, it just means your E1 menu is Auto FP, but it has no effect at 1/80 second, still regular speedlight. And the 1/80 second has no effect on flash. f/16 does though.
Two feet is measured how? Chair to the light stand? To the umbrella fabric? Reflected or shoot through fabric? Rhetorical.
But this picture is the SB-800 in a 40 inch white reflected umbrella, and the flash body is intentionally two feet from the nearest arm of the chair - just for spite.
But the flash is aimed the other way, into the reflected umbrella, so two trips along the umbrella shaft adds about four feet. So the light is six feet from the near arm, and about eight feet from the more distant parts of the chair. Six to eight feet is nearly one stop difference in the light across the span of the chair.
I think most chairs are really going to need two flash in two umbrellas. You could set the flash back farther to even the light out, but not at f/16.
It would surely mean more to see YOUR picture of YOUR situation, but in absence, I'm just trying to substitute one.
The top one is TTL mode, the second one is TTL BL mode (no other adjustments of any kind).
D800 Matrix metering and SB-800. These are 1/80 f/16 ISO 200 at "two feet" (back of flash body to the nearest point of chair). But the loss is well more than six feet, due to also the reflection loss at the fabric. It is a dark picture, which is just how TTL meters sometimes (this one used a hot shoe extension cord to the flash). This one seriously needs two flashes and umbrellas, to even it out.
I don't really know what the TT5 can do about mode, but you can select Spot Metering on the camera. Since you are not in bright light, so the dim underexposed ambient situation will not be affected by Spot (it's too weak for us to care what it meters). The flash system never uses Spot metering anyway (or Matrix - those are about ambient, and flash has its own central system), but Spot will change TTL BL to be TTL mode, and you should see a little brighter image. And TTL mode also rules out any harmful D-lens effects.
We certainly need to understand Spot metering to use it in any bright ambient situation where it definitely has major effect. But when the ambient is so far below the flash (indoors), Spot has no effect except to convert the flash system from TTL BL to be TTL, often a plus. And in Manual camera mode, Spot could not adjust the camera settings anyway.
Saying... Spot metering will change TTL BL to be TTL mode, often good effect, and indoors (dim ambient) in manual camera mode, it has no other effect. This is worth considering (indoors).
I don't think this result is so unusual for TTL, metering just varies, and we just compensate flash as we see is needed. That is what photographers do. We can moan and groan, but simply just fixing it helps a lot more.
The idea is to simply do what we see we need to do. It's our job. Make it look like you want it to look. It is very unrealistic to imagine the camera (a dumb computer) can always get it right.
This one case is not power, not quite yet. The SB-800 flash has a little more power, able to do up to 1.67 stops more compensation in this second case, which helps it. I did not show it here, but it has enough more to help.
I do usually try to get flash exposure closer, but it can also be adjusted in raw. I did not here.
