Raised in the jdk-dev ml:
https://mail.openjdk.java.net/pipermail/jdk-dev/2020-September/004727.html

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For scripts Deseret, Osage, Old Hungarian, Warang Citi,
Medefaidrin, and Adlam, for strings with upper- and
lowercase variants of the same letter, the following
code fails:

Pattern pattern = Pattern.compile(lower, Pattern.CASE_INSENSITIVE);
Matcher matcher = pattern.matcher(upper);
assertThat(matcher.matches()).isTrue();