Is it undesirable to publish a 2019 paper in a 2020 issue?

In the present day, publication year is essentially irrelevant. What matters for the job market is the acceptance date. Usually publishers put the article online within days of acceptance. At this point people begin citing it. If the paper is not assigned to an issue until the next year, the only effect will be that people citing the paper will be annoyed by copyeditors asking what the publication year and issue are.