Really? That's very odd. Episode 6 ends with 'to be continued'... That doesn't normally imply 'to be continued in the next series in a year's time'... Plus, the previous seasons have all been eight episodes. Why cut it to six all of a sudden - and with all the episodes of different lengths... All very odd, and no explanation that I can see anywhere.
Yes, when Amazon took it on they commissioned season 3, then seasons 4 and 5 together shortly afterwards. But something strange seems to have happened to how these are getting aired... Ah well, enjoy it while we can I suppose