If the Custom String widget is inside of a Dataview, you would need to trigger a refresh of that Dataview in a microflow. If not, you’ll need to somehow reload the page. You can accomplish this using the Microflow Timer widget on that page to refresh the page periodically.
If the information you are updating is not updated from the page you are on, I think the microflow timer is your best option.
I think this might be part of the reason: https://forum.mendix.com/link/ideas/70
I’m seeing this issue also in a List View. Does that make sense?