This has been a frustrating issue. Personally, I was using the folder item count for my smart folder which tracks flagged emails. I've since found somewhat of a workaround by using the "Snooze" functionality on emails, which allows an email to be hidden from the inbox until a certain date/time. You can then add the "Snoozed" folder to your Favorites. This folder shows the item count. It's not ideal and it won't help everyone, but it's something for now. Isn't it a bit frustrating that the exact functionality we're looking for (item count) works perfectly well by default on the "Snoozed" folder?!