The server.properties generated when first launching the server jar does not contain the line "level-seed=". However, if the line is manually appended to the server.properties file, it will correctly generate a world with that seed and move the line to the "correct" location in the file (between "rcon.port" and "enable-command-block").
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We're tracking this issue in MC-227819, so this ticket is being resolved and linked as a duplicate.
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