Indeed the onclick microflow. It occurs when the formatstring is used in a listview with a onclick MF on this Listview. I stripped the onclick MF out of the widget (because not used). But it should be better fixed in the widget itself. I'll post it also on gitub
I've seen this behaviour with, if I'm not mistaken, the checkbox set selector widget where a microflow was triggered on an on change event, while there was no on change microflow configured.
So probably this issue originates from a custom widget. Maybe this helps you in finding the root cause.