FULL PRODUCT VERSION :
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.3.9600]

A DESCRIPTION OF THE PROBLEM :
According to CaseFolding-8.0.0.txt the code point 13F8 must become 13F0 by String.toUpperCase().toLowerCase(). Unfortunately it does not on JRE 9.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
jshell> new String(new byte[] {0x13, (byte) 0xF8}).toUpperCase().toLowerCase().codePoints().mapToObj(Integer::toHexString).collect(Collectors.joining())

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
"13f0" according to CaseFolding-8.0.0.txt which says "13F8; C; 13F0; # CHEROKEE SMALL LETTER YE" for this particular case
ACTUAL -
"13f8"

ERROR MESSAGES/STACK TRACES THAT OCCUR :
There is neither an error message not a crash log.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
jshell> new String(new byte[] {0x13, (byte) 0xF8}).toUpperCase().toLowerCase().codePoints().mapToObj(Integer::toHexString).collect(Collectors.joining())
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
There is no existing workaround.