No, you do not need to obtain a corrected 2019 Form 5498 or amend your 2018 tax return.
The 2019 Form 5498 is correct. Unlike the reporting of regular personal traditional IRA contributions on Forms 5498, Forms 5498 report the year in which a SEP contribution is made, not the year for which the contribution is made. It's up to you as employer to track the year for which the contribution is made by reflecting that on your tax return as you have already done on your 2018 tax return.