A change in processing...
I think reason is backlog of cases and recent holiday season. Expect to see FP notices for TSC July 28 and greater applicants in coming weeks.
My suspicion (and this is pure conjecture) is that TSC decided to change their processing flow some time in October/November due to the large application queue.
Suppose, as an example, they have to do 5-6 steps to process an application. Enter data into system, do some sanity checking, cash check, send out noa, electronically send file to DO and perhaps schedule FP etc... Prior to August, people would often receive NOA and FP on the same day or at about the same time, so the service center and DO were in near sync with their processing..didn't matter who scheduled the FP.
Now, suppose the normal sanity checking step takes long for each application, the check cashing is also delayed. We were clearly reaching the point where some applications (and checks) would have been pending for 90 days and would have expired. If that had happened, TSC would just have more work on their plate. They would have to send a check notice to the applicants, put the files in a frozen state, wait for checks and then reprocess them out of order.. This just adds more work on the same staff, breaks the normal flow of processing and adds to the mess.. So its conceivable that TSC thought a little ahead and reduced upfront processing of an application to a minimum, cashed checks quickly and are now doing a second pass on the files with the remaining verification. And since the files are not fully ready for FP processing, none of the DOs can proceed either... which would explain why we haven't seen ANY TSC affiliated DO send out FP notices for post July. If the delays had been DO specific, some DOs would have sent out FP notices by now...
I would also guess that the DOs would have been redirected to do alternative work for the past few months and wont be re-tasked to process the pending FP queue until a certain date... We should see the FP notices very quickly once they unfreeze the FP queue...
The above is purely a guess, but frankly, if I were in their shoes, I would do something like it... And although most people would rather subscribe to the "TSC is a bunch of idiots" thesis, it need not be so... USCIS knew early on they had an hot potato in their hands and had to react with appropriate process changes that improved their overall processing efficiency... (at the cost of delay to each application)...
(Anyone with a math/cs background? The above should sound very familiar