KEYWORDS must be defined on exactly one line due to ekeyword (and other tools) which use naive line-based parsing. This has been a QA policy for some time: https://projects.gentoo.org/qa/policy-guide/ebuild-format.html#pg0105. I removed most trivial cases in the tree a month ago: https://github.com/gentoo/gentoo/pull/19467. We still have some left. Fortunately, pkgcheck can now detect this (`pkgcheck scan -k MultipleKeywordsLines`): https://github.com/pkgcore/pkgcheck/issues/303. If your package has multiple defined, please drop them and define only one (the operational/effective version). Note that PMS says it is safe to not define KEYWORDS in e.g. live ebuilds: https://dev.gentoo.org/~ulm/pms/head/pms.html#x1-620007.3 as it defaults to none.
I'd assume (re-)defining the KEYWORD in an eclass is an extension of this. See bug #562464
