LATIN CAPITAL LETTER SHARP S
uc("ẞ") ⇒ "ẞ"   (LATIN CAPITAL LETTER SHARP S)
lc("ẞ") ⇒ "ß"   (LATIN SMALL LETTER SHARP S)
fc("ẞ") ⇒ "ss"

LATIN SMALL LETTER SHARP S
uc("ß") ⇒ "SS"  ←
lc("ß") ⇒ "ß"   (LATIN SMALL LETTER SHARP S)
fc("ß") ⇒ "ss"

uc("SS") ⇒ "SS"
lc("SS") ⇒ "ss"
fc("SS") ⇒ "ss"

uc("ss") ⇒ "SS"
lc("ss") ⇒ "ss"
fc("ss") ⇒ "ss"

If you reverse the uc and the lc, the two expressions will be equivalent for this example, but I don't know if it holds for all cases, and it's not something you should count on.