To answer the question; it's not "normal" for an intact male to mount a pregnant female with the intention of breeding.

It is normal for one dog to go through the motions of attempting to mount another dog for many other reasons including play.

It's difficult for anyone to hazard a guess about what's going on without more information. Information such as what they were doing at the time of the mounting incident.

Is this the first time your male has ever done this? If it is, you might want to check her anal gland to see if it is full.