I have not seen this in other versions.
I have a database, based on the mail8.ntf template, in which I have created a folder. I run an agent that finds all duplicate items and puts them into this folder. I delete the older duplicates, leaving only single copies (in this example 100 items). These all have an unread status. I then press a Ctrl-A to select all (100) of them, press the Insert key to mark them all Read. Now, I want to remove them from this folder. If I look in the status bar, instead of 100 items selected, only the number of items showing on the screen (in my case 34) are selected. I now have to press a Ctrl-A again to make sure everything is selected.
Now that I know what is happening, I can compensate for it, but the first few times I did this, I kept wondering why the folder wasn’t empty the next time I went into it.