OK, this was working earlier in the year (I literally used it for a client) and now it is not working.
This is now not working again (across multiple) tenancies, with both of the available 'Modified' triggers for list items.
The version number is staying the same (bNewDocumentUpdate), and the Modified date is staying the same (as you set that with the fields using G and your locale), so it looks like Microsoft have changed the trigger.
We shouldn't have to be flip-flopping our (no/low)code to the whims of Microsoft. Plus, if you put this in place for a client (or where you work, and you move on), and they then wish to add additional modified based Logic Apps or Flows, they're going to have *real* problems.