When we flush the upstairs towlet it causes a large bubble in the bathroom downstairs

Welcome to AskMeHelpDesk. Let me explain. You have a partial blockage and every time you flush upstairs the discharge hits the blockage and bounces back sending a bubble of sewer gas ahead of it. There's your bubble. The solution would be to pull the bubbly toilet and snake the line from there. Let me know how you make out. Tom