It's not. Uppercase of ß has always been SS.
Before we had a separate codepoint in Unicode this caused problems with round-tripping between upper and lower case. So Unicode rightfully introduced a separate codepoint specifically for that use case in 2008.
This inspired designers to design a glyph for that codepoint looking similar to ß. Nothing wrong with that.
Some liked the idea and it got some foothold, so in 2017, the Council for German Orthography allowed it as an acceptable variant.
Maybe it will win, maybe not, but for now in standard German the uppercase of ß is still SS and Unicode rightfully reflects that.
I don't know how or if systems deal with this, but ß should be printed as ss if ß is unavailable in the font. It's possible this is completely up to the user.
[1] https://unicode.org/faq/casemap_charprop.html [2] https://www.rechtschreibrat.com/DOX/RfdR_Amtliches-Regelwerk...
Where does the source corroborate that claim? Can you give is a hint where to find the source?
While in older versions [1] it was the other way around:
> E3: Bei Schreibung mit Großbuchstaben schreibt man SS. Daneben ist auch die Verwendung des Großbuchstabens ẞ möglich. Beispiel: Straße – STRASSE – STRAẞE.
[1] https://www.rechtschreibrat.com/DOX/rfdr_Regeln_2016_redigie...