Is it undesirable to publish a 2019 paper in a 2020 issue? This question is inspired by How to make sense of a 2019 paper published in 2016 journal issue? From the comments to that question, it's apparent that publishing a 2019 paper in a 2016 issue is inconvenient for the author. This question asks the reverse: what if the journal is so "frontlogged" that it publishes its 2019 papers in 2020 issues?

I have handled such a journal at one point. The journal was receiving and accepting lots of submissions, so many in fact that it was publishing issues ahead of time. Previous editors were reluctant to increase the issue count because they feared this stream of papers would dry up, and if the journal ever struggled to fill its issues it would lead to problems like in the linked question, where 2019 papers are published in 2016 issues. The upshot was, by the time I took over the journal it was more than a year ahead of schedule.

Is the journal being ahead of schedule a problem for authors? What about librarians?
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.