In fact, the name "extended" grapheme cluster should give it away. There was already a major revision to UAX #29 so that the original version is now referred as

In fact, the name "extended" grapheme cluster should give it away. There was already a major revision to UAX #29 so that the original version is now referred as to "legacy". Your example is exactly this case: the second character, U+0E33 THAI CHARACTER SARA AM, prohibits a cluster boundary now but previously didn't [1]. [1] Relevant specifications: https://unicode.org/reports/tr29/#SpacingMark https://unicode.org/reports/tr29/#SpacingMark and https://unicode.org/reports/tr29/#GB9a https://unicode.org/reports/tr29/#GB9a