The short answer is : because you are using Merged Clips, the BRAW Studio Panel is opened and the selection is not freezed. Check the workarounds below to know what to do !

The detailed Reason Behind the "Bug" : Merged Clips

In the BRAW Studio Panel, we need to get the filepath from the current selection (which can be from the timeline OR from the Project Panel).
If it is a BRAW file, then we accept it and it can work with the panel.
The problem comes with Merged Clips : Premiere Pro doesn't give us the different filepaths inside the Merged Clips selected.
So a workaround for this in our side is to export the Merged Clip as FCP XML (which is internally considered as a sequence but which we can't directly see what is inside), and then take the filepath from this XML.
It is done quite quickly, but we can't avoid Premiere Pro to save the project while exporting the FCP XML .. (That's why you get a "Save Popup").
We only do it once per Premiere Pro project for each Merged Clip Project Item, otherwise it would save the project each time you select the item.

Solutions / Workarounds

Solution 1 : Avoid Merged Clips

One of the best workaround is to not use Merged Clips. I imagine that could be a problem for some editors which are used to work with Merged Clips in their workflow though.
In our opinion it makes Premiere Pro less safer in term of stability .. For example there are problems while using the Project Manager (project consolidation). Also there is no "Un-merge" solution ...

Solution 2 : Select all Merged Clips first

You can choose to select all your clips first. After this if you don't add new Merged Clips there will be no save popup due to Merged Clip anymore for this project. It could be easier in the Project Panel, but can also be done in the timeline (hit Ctrl(Cmd for Mac) + A !)
In that case, you will see all the "save popup" multiple time but all in once. It could take some time depending on how many BRAW clips you have.

Solution 3 : Use the "Freeze Selection" button

You can freeze the selection in the BRAW Studio Panel. In the first Source Settings tab, there is a button available when you select a BRAW file : "Fix Selection".
Then, when clicking on other clips the selection will not be updated, and so you should not get the "save" popup anymore.

Solution 4 : Do not open the BRAW Studio Panel

If you open Premiere Pro / Your project and don't open the BRAW Studio Panel (in case you don't need to access its features), then the selection will not be updated in this panel and so no "save" popup. Note : if you just close the panel, it could not be enough (because Premiere Pro doesn't unload the panel when it is closed) and you will need to restart Premiere Pro.