After visiting multiple OnePlus and XDA forums, I've read multiple explanations.
Many people have contributed the problem to adhesive used in the OnePlus One, and some have suggested that the glue needs time to "cure".
An email from OnePlus support stated that
''"Our screens are lit from the bottom instead of from underneath, and to minimize bezels we place the light sources very close to the edge of the screen. To prevent excessive light leakage, the bottom edge is designed to be slightly opaque, which is why it appears yellow from certain angles."''
The teardown for the OnePlus One can be found [guide|26484|here], but unfortunately the yellow band issue wasn't brought up.
If the glue simply needs to cure, then hopefully your problem will be solved with time. Otherwise, if the screen is designed to block light bleeding near the bottom, that may be an unfortunate side effect of the design.