Why does upper flr. hot water faucet INSTANTELY go cold as soon as any lower flr. hot water faucet is put on?

There is only one brand new,bigger hot water tank. Not an instant or dual. There is no return loop. no mixing valves. no plumbing pipe changes. no pump. When it happens…the hot pipe off the tank stays hot, the cold stays cold.Pressure stays great at all times..