First off, I am extremely happy overall with the updates to the Attachment Viewer. I have been working on a personal project for a year now related to the September 11 attacks and mapping out every photo taken in the city that day and the Attachment Viewer is exactly the kind of tool I pictured in my head when I was tired of working with the "Google My Maps" tool and lack of features.
With the change, I've run into a few immediate issues:
1.) When clicking on a point on the map, it will reset the view to a very distant zoom:
Example: Clicking any point here --

Switches the view to this distant height:

This did not occur prior to the update and it makes it difficult when constantly clicking between points on the map.
2.) Hovering over images in the attachment viewer gallery no longer shows the name on the image.
This one is more Quality of Life, but I am not sure why this feature is no longer there. Now if you hover over a photo in the gallery view, it will just highlight the point on the map and shows the name directly on the map. Seeing the name directly on the image in the attachment gallery view was very helpful when quickly scrolling through images.

3.) Mobile - Unable to properly render tools and settings.
The mobile interface is miles better than it was previously and I am very thankful that you can now scroll through the gallery at the same time as viewing the map. This was very difficult to explain when showing this to other people.
The issue though now is after this update, you can no longer efficiently use the tools.
At the top-right of the screen, you can see the filters, layers, legend, and selection tools.

When clicking them, they are unable to display properly.

Prior to the recent updates, these were stacked vertically rather than horizontally and could be viewed without any issue.
I also have one additional minor feedback and that is the introduction of the new "Additional locations" feature which appears at the bottom of the screen after clicking on a point.

My only grievance with it is that this shows images from around the 100 last points to be added. Because of this, if no changes are made, it's constantly showing the same 100 attachments regardless of if you're clicking one of many thousands of other points. I feel it would be improved if it could behave the same as the normal attachment gallery view that shows attachments in ascending/descending order based on your current view. That way if you want to continue scrolling through attachments from a given viewpoint, you don't need to close out of the current point to move to the next.
And because of this behavior, I would also have preferred if this were to be an optional toggle rather than a strict requirement to be included.
In any case, I am overall a huge fan of these upgrades!