scanf_s can not accpet a number follow by e or E, but other letter works, why

Question

I am a beginner of C coding. I met a strange problem recently, I wrote a statement like this:

scanf_s("%lf",&num); 

here num is a long double type variable that I declared before.

Every time I type in like "1E", "1e" or "1ea", an 'e' or 'E' followed by a number, then num will get nothing.

But if I type in like "1a", "1b" or "1d", any other letters followed by a number, then the number will be taken correctly.

It is so weird. Would anyone please me with this, thank a mil!