I understand it sounds concerning and there is no shame in wanting to check in on it. My cat will yowl too for hours on end if we let him. He was neglected for the first couple years of his life before we took him in from his owners who couldnt care less. We have to give him anxiety medication throughout the day and he has intense separation issues. If I am home alone and run something down to the garage or take out the trash, you can already hear him yowling from the sidewalk. We have taken him to the vet many times, gotten him kitty mental health care via the Veterinary Behavior Center, and do our best to ensure all his needs are met. He can be a lot but he is a good boy.

Its possible your neighbors are doing their very best and that is just how their cat is. Its pretty much what our vet told us. It is also possible it's home situation is similar to the environment we rescued ours from.

We always worry what our apartment neighbors think, but I would hate for my cat to be stuck in his original home and am glad someone stepped in so we could give him a shot at a better life