There was an interesting discussion about comparing models here:
which might help illustrate some techniques. One method I would try nowadays is simply save out the ‘before’ and ‘after’ IDF or OSM files, and just hand them off to an LLM and ask it to spot the difference(s). I’d bet you it’ll find the difference(s) in a few seconds. Once you know what the difference is in the IDF/OSM file, you can then trace down the issue in the Grasshopper file that is causing it.