Incorrect reversal of the U+0489 character

[lunakurame]

I've got a string with this character:
҉ U+0489 COMBINING CYRILLIC MILLIONS SIGN

Before reversing: te҉st te\u0489st
After reversing: ts҉et ts\u0489et

I might be wrong, but I expected tse҉t tse\u0489t instead. Is there a reason why it behaves like this or is it just a bug? I found it when my unit tests failed while checking my code using random zalgo examples.

[mathiasbynens]

This is an example of a symbol with the Grapheme_Extend property. IIUC they can only really follow Grapheme_Base symbols, but for esrever’s purposes we could probably just treat them like regular combining marks.