This is something I've never experienced on any version of Alfred (since Alfred since March, 2011).
If I open Alfred and type 2 and then hold SHIFT to type * to multiply I get an error ding, the sort you get when you make an error in Terminal, etc. The message below says "Please enter a valid expression" in the Calculator preview below, as if I've told Alfred, "OK, I'm finished; give me the answer to 2 *". If I complete the expression by typing another 2 (so, 2 * 2), I get another two dings, even though Alfred recognizes that there is now a valid expression in the input field and gives me the answer in the Calculator preview below.
System Info
Catalina (10.15.1)
Alfred 4.0.6