As the article says "The discovery of this planet shows that planet formation and evolution proceeds in unusual ways not anticipated from studying the solar system," David Armstrong, the lead author of a new study reporting the discovery of TOI-849b, told Space.com via email. "There is a huge range and diversity of planets out there, with more found every week, and if anything is becoming clear it is that we should not be surprised at strange new discoveries,"
Very good observations here. TOI-849 b is considered about 39 earth masses with mean density some 5.2 g cm^-3,
https://www.nature.com/articles/s41586-020-2421-7, 'A remnant planetary core in the hot-Neptune desert'. My observation, I like the failed gas giant answer. Exoplanet discoveries like this present various challenges and problems concerning gas disk modeling for solar systems and planet evolution.