britbox captions black out after about 40 minutes

I recently had my set top box replaced. Since that was done, I've noticed when I watch a show on Britbox (through the Xfinity Apps menu) the closed captions blacken out after about 40 minutes.  This did not happen with the previous box. If I leave Britbox and return to watch the rest of the show, the captions come back on. I can't find anything in the community forum on this. The troubleshooting tips for box malfunction have been done repeatedly and they don't affect this problem.