These are being caused by tests that send grapheme clusters represented by more than one unique code point, including emojis and non-English composed characters.
<rdar://100588599>
Pull request: https://github.com/WebKit/WebKit/pull/7323
Committed 259092@main (bd0b584ba907): <https://commits.webkit.org/259092@main> Reviewed commits have been landed. Closing PR #7323 and removing active labels.