IJAMinecraft

I can confirm that the same issue still happens in 1.21.8.
I summoned a text display using the following command, with the translation key ija.oneblock.tl.c defined in both English and German inside a resource pack embedded in the world:

/summon text_display ~ ~ ~ {alignment:"center",billboard:"center",text:{translate:"ija.oneblock.tl.c"}}

I summon the text display while the game is set to English and the English translation correctly gets shown. If I switch to German, the English translation still gets shown. However, once I leave the world and rejoin, the German translation correctly gets shown.

This still happens in version 1.21.8. I summoned a see-through text display using this command and it renders behind villagers and chests:

/summon text_display ~ ~ ~ {alignment:"center",billboard:"center",see_through:true,text:"VeryLongTextToTestSomethingOut"}

The villager and blocks were placed after the text display was summoned, but the same result happens if it’s the other way around with the text displayed summoned last.

The text display also renders behind the particles of barrier blocks: