Some backlit displays control brightness in zones larger than individual pixels. A bright subtitle can illuminate a zone containing nearby dark picture areas, creating visible glow or blooming. Viewing angle, picture mode and dimming behaviour can affect it; the source text is not necessarily defective.
Identify a display interaction
Pause on a dark scene with a bright caption and compare another source or menu with similar contrast. If the glow follows bright objects across otherwise dark areas, local backlight behaviour may be involved. A phone camera can exaggerate the effect, so judge it from your normal viewing position.
Try supported caption styling
A slightly dimmer text colour or a suitable background style can reduce the contrast demand where the player allows it. Keep the text readable and do not remove needed captions merely to improve a black-screen demonstration. Burned-in subtitles cannot usually be restyled by the player.
Compare dimming settings carefully
A lower local-dimming option may reduce one kind of glow while changing black levels and highlight impact. Test normal scenes as well as the difficult caption moment. Choose a comfortable compromise for the actual room rather than expecting one setting to remove every physical limitation of the panel technology.