Oh now that is weird - but very useful info! It suggests that rather than there being “nothing” at the edge of the road in the volcano there are actually “invisible walls” just on that one section in particular (since I’ve not seen this bug happen elsewhere). I wonder if they added invisible walls as a “hack” to stop people riding off the edge of the road (as noted in this post Bug: Riders riding on terrain, off-piste and several others) since that would cause them to ride through lava in this case or “float” in the air since the road edges drop away sharply.
It would certainly make sense for them to be related, especially since I don’t think I’ve ever seen anyone riding beyond the edge of the road in the volcano, unlike on many other sections of road. It would be another case of “building a bodge on top of a bodge” on Zwift’s part - adding invisible walls rather than just making the code restrict riders lateral movement within the defined road bounds in the first place 
They do make headaches for themselves with stuff like this.
I think you may have cracked it 