At the end of some released "unrated" films, there is an MPAA seal of approval for strange reasons. This seal is usually issued after the film has been rated. Here are some examples below, all of which have never been rated since. The first one is from The Postcard Killings (2020), and the second one is from Plus One (2019).
The MPAA may not give a film a rating but the number is issued to indicate that is was submitted for review
I found this from a link in a previous question regarding MPAA numbers
The Motion Picture Association of America (the MPAA) is responsible for assigning these numbers. It is part of their film rating service. Any film can be submitted to the MPAA for rating (the G/PG/PG13/R/NC-17 ratings Americans are familiar with), for a small fee. Any film rated by the MPAA is issued a unique number.
In this case, a number would be issued but not actually rated with any specific code.
Also, on a separate link
For each movie film, displayed on screen at the end of the credits of every USA released movie film, the Motion Picture Association of America (MPAA) has issued a unique number to identify the title.
If a film has not been submitted for a rating or is an uncut version of a film that was submitted, the labels Not Rated (NR) or Unrated (UR) are often used. Uncut/extended versions of films that are labeled "Unrated" also contain warnings saying that the uncut version of the film contains content that differs from the theatrical release and might not be suitable for minors.