Well, a stationary bike shouldn't count steps unless you are swinging your arms instead of holding the handlebars -- unless it's the type of bike where the handlebars move as well. You don't, by any chance, also have your phone in your pocket while riding? The phone counts steps differently but combines the count with the watch. Maybe the update added a different counting method for the steps, as many people have been complaining their steps aren't being counted when they are pushing a stroller or shopping cart or otherwise have their watch arm engaged. There's not going to be a perfect counting method. This is from the health app:
