I attach a very simple file which demonstrates this, but in words the problem is this:
An extrusion can be split or trimmed with a line regardless of where the line is as long as it crosses the extrusion in the viewport used for the command.
A 2D entity however, can be trimmed as above, but it can’t be split if the line is not actually intersecting the entity.
Why? It drives me nuts, especially as I can see no logical reason why it should be so…?? It just feels like an oversight that both these similar commands don’t follow the same rule…?
[In the file, with the right viewport being active, any of the lines will TRIM the objects, but the red line will not SPLIT the circle, because it doesn’t actually intersect it, even though a similar non-intersecting line will split the extrusion.]