Bolding of self links was provided via <strong> elements until a change to MW core in 2017. Since that change, skins are required to provide styling for the self links to appear in bold.
This has been done in Vector, Monobook, Timeless (patch), CologneBlue (patch) and Modern (patch). Minerva has chosen not to bold self links (see discussion in T181472).
Other skins in Gerrit (list) should be checked, and similarly fixed if needed.
Original task description
Probably as a result of T181472: "a.mw-selflink" links miss styling and related, self links in Timeless no longer bold themselves. (They do seem to provide hover CSS however, which should probably also be removed.)