There are always times of stress in the cockpit, it's just that some people are better at hiding it than others.
Ideally all FI's should be steely-eyed and unflappable, but reality isn't like that.
Very few FI's that I've ever met are nervous people, if they were they wouldn't last very long.
Have I ever felt scared by the actions of a student? Very occasionally, but have I ever been scared by the actions of a supposedly licenced PPL? Oh yes on numerous occasions!
While we do notice most things coming a mile away, FI's aren't clairvoyant and sometimes people do things that you couldn't possibly forsee.
I look pretty calm when I'm in an a/c, but I am primed at all times to do something. I have had too many people do absolutely stupid things with no warning.
No-one could seriously tell you whether what your FI is doing is acceptable as all we hav is your comment that you are "rational". Believe me when I say that some people's idea of rational is a darn sight different to mine!