For the word “Monsieur”, I found that “on” has no pronunciation.
Is it normal in Franch？
Yes, the “on” in “monsieur” is an exception. It’s pronounced like “me cieux”
Exaclty, that word originally was “mon sieur” which ment “my sir”. Pronunciation has evolved but not spelling, And in plural it’s “messieurs”, “my sirs” 