Hi
I know this sounds strange, but it may help your behaviourist if you can record an attack, so they can see it.
When I first got Charlie this was something that the vet and behaviourist felt he may have.Thankfully in his case it was fear reaction, and he did not need medication.
My friend had a dog who reacted suddenly for no apparent reason, it came to light she had a shadow on her eye that affected her vision and caused her to "spook"
i hope you manage to get the right help for Millie