Sometimes subtitles are baked into the video especially when they're translations of short periods of someone speaking a foreign language, so you'll see both the subtitles that are part of the video along with text overlaid on top of the video. The next time it happens try disabling subtitles to see if it allows you to see the text that's part of the video.
If that's the problem, I don't really know of a way to fix it other than constantly enabling and disabling subtitles since Roku doesn't give you any way to position subtitles. You can play with the different settings under Settings->Accessibility->Captions style for color and transparency, but I don't know if all channels/apps honor those settings and at best you'll still be trying to read text that's under other text.
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