HOUSTON—Exxon Mobil Corp. on Wednesday offered its first concession in a new contract proposal to locked-out workers at its Beaumont, Texas, refinery and lube oil plant to include some seniority protection. Exxon on May 1 locked some 650 workers out at the 369,000-barrel-per-day refinery, replacing them with temporary workers and saying the company was afraid a strike might lead to disruptions to production. “The amended offer, which comes after months of good-faith bargaining meetings, provides further support to our represented employees’ concerns about security and seniority,” spokesperson Julie King said in an email. The leaders of United Steelworkers union (USW) local 13-243, which represents the locked-out workers, said Exxon’s new offer fell short. “Although these changes are recognized as movement by the company, the overall 40-page comprehensive offer still leaves much to be desired,” the leaders said in a statement issued Wednesday night. Exxon has not withdrawn proposals that would …