This is totally impossible in theory, but it happens in simulation. Here is the thing:
The attached image shows the circuit I built and the current curve when R1=0 which is correct (it is the curve that it should be).
HOWEVER, when R1 changes, the shape of the current curve changes as well, which is QUITE confusing. In my point of view, the current curve will shift when R1 varies but its shape should always be the same.
Therefore, I cannot understand the second current simulation result. Could you please give me some hints? I am completely confused.

 

You're not the only one.
What is U1?

 

You're not the only one.
What is U1?

That is a module I built by myself depend on its IV curve. So you got the same problem? Have you figured it out?

 

How can we get the same problem if we don't have the spice model for your mystery module?
Welcome to AAC!

 

That is a module I built by myself depend on its IV curve. So you got the same problem? Have you figured it out?

No.
My confusion was about the nature of U1, not that I duplicated your problem (which I can't since I don't have the model for U1).

 

That is a module I built by myself depend on its IV curve. So you got the same problem? Have you figured it out?

So you don't think that the behavior of the circuit might just depend on what is in this module that you built? And you expect other people to be able to replicate your results without telling them anything about what is in this module that you built?