I think these were copy pasted from text-align, which needs it for the hasExplicitlySetTextAlign flag, which text-align-last does not have.
<rdar://problem/95131415>
Pull request: https://github.com/WebKit/WebKit/pull/1514
Committed r295541 (251546@main): <https://commits.webkit.org/251546@main> Reviewed commits have been landed. Closing PR #1514 and removing active labels.