Starting from version 2.2.11-r1, if either "mysql" or "mariadb" USE flag was specified, the package depends on dev-db/mysql-connector-c or dev-db/mariadb-connector-c. Consequently, app-misc/sphinx now indirectly conflicts with any kind of mysql server — dev-db/mysql, dev-db/mariadb, dev-db/percona-server, ... Instead, app-misc/sphinx should depend on virtual/libmysqlclient, which is responsible for choosing a corresponding source of connector files.
https://archives.gentoo.org/gentoo-dev-announce/message/618f39bce863911d8955ee9f83636ca4 How do they conflict (with latest versions in testing)?
Thank you for the link, I wasn't aware of the upcoming changes. Just double checked ebuilds and their dependencies — I have a mysql server with USE="client-libs", and it appears this flag is the reason why both connectors actively refuse to coexist with the server. Sorry for wasting your time and thanks again!