Pure speculation here (and I'll explain what I'm thinking), but let me start with a question.
Is there anyone in the US with a Safety Score of 98 or higher that is also not currently an active FSD Beta tester (you've hit the button, but you aren't in yet) with software newer / higher than 4.5.3 that also has their software update preference set to Advanced?
For example, I have all the above mentioned ..... I've hit the enroll button, have a current SSB of 99.22 for the past month and a 100 over each of the past 8 days, have my software update set to Advanced, but haven't had 4.5.5 or higher roll out to me yet. What I'm speculating is that (and I can only hope I'm right) folks are stuck at 4.5.3 because they will get FSD 10.11 Beta sometime in the coming days and having 4.5.16 would have impacted that.