"sol diesis", but I believe you refer to reading it while doing solfege with chromatic alterations, where you would read Sol# as "Si", right?

The answer is: I have no clue, I think we do not actually account for flat and sharp when doing solfege.

Or better put: "si" was changed to "ti" in UK to be able to read chromatic alterations without ambiguity. Most of the latin and slavic world ignored this, AFAICT.

But full discosure: I know very little about music, music theory or music history, I just have vague memories of reading about it.