I've just been trying to use Miro for Story Mapping using the Story Mapping template. It was looking good to start with but after a while when I came to need to organise Activities and items on the backbone row it all just fell apart and I had to give up.
The behaviour of the Activity and Backbone row items basically breaks the entire usefulness of the User Story Map template for me. The way I story map, I have Activities at the top, then items on the backbone row form the 'walking skeleton', these are the minimum set of non-optional stories that enable the system to work. To start with items under the backbone might be details or sub-tasks of backbone items, but further down the map, perhaps several release slices down, tasks at that location still relate to the activity, but may not relate directly to whatever task was on the backbone row. These later tasks might represent alternative user journeys through the system, or quality of life improvements for the activity.
The problem is that the Miro template however assumes that anything in a column is intimately releated all the way to the end of the story map, so if you move an item on the backbone it automatically moves everything below it. You also can't delete anything on the backbone without deleting every task below it. Similarly with the Activities. Deleting an Activity deletes every task between it and the next Activity. You can't add an Activity to the top of an existing column, any new Activity is instead added at the end of the existing backbone. Attempting to move an Activity separates it completely from the rest of the Story Map and seems to create a new frame.
As the User I know where I want to put things, and if I want to move or delete things in bulk I already have the ability to do that using CTRL click, but the built in assumptions around how Activities and items on the backbone row should work completely prevent me from doing what I need to do. I need to be able to move Activities and items on the backbone row to where I know I want them, independently of what might happen to be under them.
So my suggestion is this, just give the User the ability to put things where they want and the system will be far more flexible and useful than loading it down with all these built in assumptions that don't provide the User with any extra functionality but which instead prevent them from doing basic things that they need to be able to do.
On a unrelated note I also had issues moving the story map. Two fingered dragging to move the map sometimes permits moving both horizontally and vertically, but commonly only permits moving on one axis at a time, with no clue as to why or how to switch between the different modes. So you end up just repeatly attempting to move the map around until it eventually (seemingly randomly) allows you to move the map on the axis you want to.