According to the Oxford English Dictionary, "invasion" means:
an instance of invading a country or region with an armed force.
According to the Oxford English Dictionary, "armed force" means:
a country's army, navy, and air force.
According to the US government,
The Army, Marine Corps, Navy, Air Force, Space Force and Coast Guard are the armed forces of the United States.
Based on those official definitions, including the one given by the US government, it seems apparent to me that the 1999 bombing of Yugoslavia by NATO was also an "invasion". I would imagine that at least some sources, official or not, would follow these definitions and define the operation as an "invasion".
However, after searching multiple keywords on Google, I find that no source has defined or used the word "invasion" to describe the Yugoslav war. It is kind of weird that all media holds a similar view on the definition of a war. Are there any reasons behind this? Is it because there is a negative connotation attached with "invasion"?