Putin knows Biden / Harris will act if he does not. He can't affort Armenia hosting US military base.
Its ok if Putin acts. None can stop him doing so. It will hurt AZ but it will make Turkey much more powerful as it will be forced to increase its own deterrents.
In a Russian invasion of AZ, I predict Turkey will invade southern Armenia and connect with unoccupied parts of AZ. Both northern AZ and southern ARM will be occupied. one by turkey one by Russia. Pakistan will open bases in AZ and provide a nuclear umbrella. Russia will be more isolated and Turkey Russia Cold War will start.
I think Putin knows this and will have to think very hard before he invades AZ.
Biden can also invade AZ when he opens a military base in America's close ally Iran, or when US occupies the dardanelles and controls what ships can travel through and which can not.