Title and subtitle text has black background

Its not a bug 😃

Thank you for your report!
However, this issue is Working as Intended.

Added a background to title and subtitle text with adjustable opacity. Opacity can be changed in the "Accessibility" tab in the Settings menu

https://feedback.minecraft.net/hc/en-us/articles/360043896251

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Thanks for referring me to this place. How can I solve this problem of the letters with black background?

Go to Settings > Accessibility and set the "Text Background Opacity" slider to 0%.

I don’t think it works with all texts except for the title and subtitle bars. There is also text background on controller tooltips and item names but does not work with the text opacity. Is this another issue or another solution?

That's another issue, which is being tracked at MCPE-79331.