Bricks of prefixes thought long gone commonly appear out of the blue and I assume it is just the way they are pulled from the depots. Many times when ordering bricks you think that your prefixes are progressing and then suddenly your back to prefixes that you were getting a year ago or even longer.
I concur: the releases of the various BOC prefixes does not appear to occur in descending alphabetical order any more than the numbering of notes (which seems to be getting more a more jumbled in many bricks). That's what I have found the past few years- maybe others have seen a different pattern. for $5.00- I just got an HPM bundle recently (before that: HPK, HPW & HPF). They seem very scrambled (and I got a run of new BTZ $10 with some BFK's earlier this year). Those weren't sealed bricks but I am pretty sure BTZ were being released over the past year as I found new ones over and over again.
I found a very high numbered HPB in an HPF brick and reported it but it has not yet been confirmed (I am assuming other brick hunters have not found one either). I'm holding onto it just in case. IMO: it may be a good idea to hold onto an unusual prefixed note that one finds in a brick (of a different prefix) for a little longer than usual just so you do not spend a later confirmed insert.