Maybe you didn't refresh the page in your browser.
To be honest, it naver happened to me, and I placed bids for more than 2000 job for sure.
Nobody is trying to cheat you or trick you. Upwork has millions of freelancers, many of whom are anxious for work and spend their day hovering over the jobs feed in order to get their bids in as fast as they can. It's entirely possible for 20-50 other people to see a job - just as you did - and place bids at the same time as you.