Astronomer here, the "life detection" on K2-18b was dimethyl sulfide (DMS) which may be ̶I̶s̶ ̶a̶n̶d̶ ̶r̶e̶m̶a̶i̶n̶s̶ a marker for life. What you get from the James Webb Space Telescope (JWST) is raw data that needs to be treated and calibrated to some extent to be usable in scientific study. This is called data retrieval.
However, the lead scientist on this paper claiming they found DMS basically used his own very specific way to do it and found very very weak signals in that way. Other scientist tried to both reproduce it in the way he did it and also with their ways to retrieve the data, but couldn't find anything. So it turns out, it was simply a non-detection.
Edit: It might be the case that DMS can be produced abiotically (scientific works of this year) as chosensilence pointed out correctly.
My main point is, that the DMS detection itself was a non-detection in this case