In my experience, OnChange microflows fire when you change the contents of a field and lose focus on the field. If you change the text in the textbox, and then click somewhere else on the page, does the microflow execute?
Are there any other input fields on the page? An on change microflow doesn’t run when there are validation errors on the page. What if you set validation to none or ‘only for this widget’ in the microflow settings, just below where you defined the on change microflow?
Here are the properties of the textbox that I have.