It is For example, on a narrow ledge each has to take a turn to jump over a gap. It seems that the ones that are last in line run the highest risk of getting left behind. It is like their window of opportunity has passed. I could be mistaken though.
You are correct. But it is pathing. Basically, the pathing algorithm fires for each party member X number of times accounting for each characters' physicality along that path in relation to the others. Any result causing two or more characters to occupy the same space at the same time means a fail and re-calculation. If, after the Xth time, the code can't find a path that is clear, it stops the character where they are. So, as an example, if the destination is a smaller spot, it is more likely to not be able to find an open space/path for one or more characters. On the other hand, open areas are less likely to have this issue, so it isn't seen (much) there.
Or I imagine that is the way it is coded.
I imagine that adding a post processing 'final attempt' after a short delay will likely resolve the issue. But I don't know the code well enough to say if there are any other limiting factors which might prevent this.