Setting the "text_opacity" NBT tag in an item display entity to anything less than 4 seemingly renders its text's opacity as 255, and setting it to 4-25 seems to render the text as completely invisible.

Steps to reproduce:

• Summon a text display:

  /summon text_display ~ ~1.5 ~ {text:'"Text"'}

• Set its opacity to something low but still visible:

  /data merge entity @e[type=text_display,limit=1] {text_opacity:26b}

• Lower it by 1 down to 25:

  /data merge entity @e[type=text_display,limit=1] {text_opacity:25b}

• Notice the text is now completely invisible.
• Set it to 1 where it should barely be visible:

  /data merge entity @e[type=text_display,limit=1] {text_opacity:1b}

• Notice the text is fully visible.