It is not a bug. That very character was submitted to Unicode by none other than DIN (the German national standardization organization) [1] and it was clearly indicated that the capital eszett should be encoded without case folding to the original eszett (p. 4). As others mentioned the capital eszett is allowed but not enforced, so DIN didn't want to break the original case mapping between ß and SS. And because of the Unicode stability policy [2], it can no longer be made into a case pair even though DIN changes its mind.
If you read the official announcement for the German language (cited in the wiki article) there it states that SS is the capitalization of ß, but ẞ instead is permitted. So the standard Form is double S.
Question: is there any reason to expect that the double s form will be replaced, or is this a more of a permanent quirk of a language adopting quirks from their neighbors?
Difficult to say. The double s Form has to stay (for small and capital) to not deviate from Swiss German too much (has to stay compatible). Generally older people think capital ẞ is ugly, but maybe this will change over generations. Best bet (for me) is that all ß fall out of favor and Germany goes the Swiss way. Let's talk in 60 years again.
When I first heard about it a few years ago I did indeed think that it was ugly-looking and the whole idea rather pointless.
I've recently re-discovered it, though and somehow this time around actually taken a liking to it, and have smuggled it into a few reports I've had to prepare since then.
