Gray characters in subtitle rendering?

Hello!

We’re working on getting a VOD app running, using MPEG-DASH video files and SMPTE-TT subtitle files.

Subtitles are working, but characters occasionally render in a broken way. Specifically, a given character is rendered in gray instead of white. See the following:

http://imgur.com/a/W466v

Here are the corresponding TTML fragments of these instances.

WATNEY: It's time to start
thinking long term.

In four years, when the next
Ares crew arrives,

Any idea what’s happening?

We’re converting into Timed Text using MacCaption from a variety of sources including STL, SCC, SMPTE TT, etc…