Per RFC 7232, ETag value in HTTP headers should be a quoted string: https://httpwg.org/specs/rfc7232.html#header.etag
Current response of the Wikibase REST API uses the revision ID in a non-quoted form, see
HTTP/1.1 200 OK Date: Mon, 11 Apr 2022 09:26:28 GMT Content-Type: application/json ... Last-Modified: Mon, 21 Mar 2022 11:38:20 GMT ETag: 10 ...
whereas it should be
... ETag: "10" ...