Probably forgot to enable refresh client or commit when you change the object indeed.
A screenshot of the microflow, including the input parameter, would be very useful here.
Otherwise, and I’d just be guessing here, you may not be refreshing the changed object on the screen after the change? Perhaps the right field isn’t displayed on the page?
Again, some additional context here would be very helpful.
I agree with Roel that you likely missed checking the “refresh in client” box on the change activity.
If all your microflow is doing is updating a value on the screen, you should make it a nanoflow instead. Then you don’t have to remember to check the “refresh in client” box because all change actions in nanoflows take effect in the client immediately!