When applying a resource pack that changes the rendertype_text shaders to replace grayscale text (such as inventory text) to another color this will also cause subtitle text to be changed as well causing flickering when the subtitle text decays to the text color being replaced.

I believe setting the subtitle text to a unique Z position (like with tooltip and chat text) would allow these packs to not affect the subtitles.

Another solution would be to have the subtitle decay function lower the alpha value instead of rgb values.