See commit message of https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/deploy/+/549245. https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/deploy/+/547311 was the earlier effort thinking this would solve it.
But, we need a mechanism to do an atomic depool + symlink + restart php-fml mechanism before repooling the server.
Right now, while we don't yet have live traffic, this isn't an issue, but once live traffic starts flowing to the cluster, this will cause user failures.