My understanding is that Tesla is switching to something called a "back end" system, which does not show manufacturing schedules to SAs. It's a regional roll out. Most likely, this is why some buyers can get tentative manufacturing dates and some cannot.
This was/is purely informational and has no affect on delivery. Even with it, the word "tentative" means just that. Those who got a tentative date were often told "plus or minus a week". I suspect this info was originally made available when the Model 3 was first being produced, and somebody who was number 59,567 in line would have a rough idea when their Model 3 would be delivered.