When some years ago I have noticed that (the effect can be drastic, f.e. with extremely slow LFO inside the plug-in), I have thought that is a bug in particular plug-in.
All scientific projects use a concept of reproducible random generators. That means even in case something is "random", if you start the program again it will generate exactly the same sequence of these "random" numbers, and so the result will be also exactly the same (till you change initial seed).
So, plug-ins COULD use project time + fixed seeds for random generators to make processed audio reproducible. But it seems like most plug-in developers was not aware about that concept... In fact I have failed to find a single plug-in which use it