I always use Calibre with my T2, and haven't seen this problem. I don't usually pay attention to the "Last Added" on the device, so am not sure what would be normal.
How many books do you have on it, and how many are you trying to add? Are you adding to main memory or SD card?
So when you 'Send to Device' from Calibre, it says the book is there. But when you go to the device, you don't see it?
Do you have it set up to use your "Tags" Column as book collections? Could it be in there?
Have you done a 'search' for the title? Is it not showing up after a search?
When I get the time, I'll plug mine in and see what it does.