I keep my mask on at my healthcare job. I do it because I actually enjoy wearing one. I think it looks cool. I got a cold a while ago. Soon after, a couple of my coworkers got sick, even though I kept my mask on throughout. Granted, it may have slipped off my Pinocchio nose during the course of the day, but I did wear it. This is evidence that masks do not stop the spread of respiratory viruses.
Your coworkers could have gotten sick from one of the dozens of other people they interface with on a daily basis.
This is not evidence of anything.