No differences in the DNA sequences for N iterations ... and yet physical and genetic abnormalities eventually appeared after N+1.

The suggestion then is that other information about the iteration of copies of copies is being passed, from the article:

  There may have been several causes behind the abnormalities that arose in later generations.

  {various potential causes discussed}

Limitations of serial cloning in mammals - https://www.nature.com/articles/s41467-026-69765-7