By far the most tragic mysterious text for me is:
Rongorongo
https://en.wikipedia.org/wiki/Rongorongo
The ability to read this language was permanently lost due to colonialist bullshit in the 19th century. The story is maddening.
It is, or would have been, of extraordinary linguistic importance, as it appears to be a completely independent origin of writing.
It's probably ideographic, and might not be true writing, but more like a mnemonic.
But I guess we'll never know.