SUMMARY

The flow control on 50% or less causes a single brush stroke to be a bit transparent and not fully opaque (color and alpha).

STEPS TO REPRODUCE

1. create a new document with a white background
2. choose the “Basic-2 Opacity” brush (any flow controllable brush that is supposed to be fully opaque)
3. set the flow strength in the brush to 50%
4. make sure the brush color is set to Black (0,0,0)
5. draw over the canvas with one stroke, overlapping itself multiple times
6. pick the color of the stroke and notice that the values will never reach 0,0,0

OBSERVED RESULT

On my end it stops with 
50% flow:
5.96046e-08 on 32bit float
0.000488281 on 16bit float
2 on 16bit int
1 on 8bit int

with 10% flow:
2.98023e-07 on 32bit float
0.00244141 on 16bit float
10 on 16bit int
5 on 8bit int

the same effect takes place with the colors switched

EXPECTED RESULT

It should be completely black with one stroke.

SOFTWARE/OS VERSIONS
Windows: 10 Pro