I don't think the unequal Vf's is a myth. Much of it has to do with the thermal coupling between the two LED's and the resistance between each one and its common point. Low resistance and poor thermal coupling makes this unstable and much more likely to result in thermal runaway.
Why would the...
Try forcing the step size to a small value in the simulation setup. Sometimes the algorithms that try to speed up the simulation get themselves into trouble.
I have found that by setting a fixed small step size and just waiting a little bit fixes a lot of things.
The circuit should work as drawn.
