That's Amazon for you. At least three or four people have noted that 5.11.x upgraded with
update.bin.tmp.partial in place. And 5.12.x is not your friend.
This is of course frustrating but the blocking folder was
never a guarantee that fw would never be upgraded. It worked for quite a while but there has been discussion at length that Amazon did have ways of forcing upgrades if they wanted to be more aggressive so it was unlikely to last forever. Which we can now confirm.
There are various ways to try to prevent upgrades. Airplane mode is by far the best. But there are a lot of other possible approaches and it's perhaps best to employ multiple obstacles to unwanted upgrades.
It's good that you kept the jb packages up to date. You may also find
this post useful.