FireTV 4K Mysterious Transcoding

[abpjf]

I'm not saying that this is the cause rather more of an observational comment, but never take published specs as gospel; it's not unheard of for a mfr to basically copy-paste specs (you're dealing with Marketing at the end stage, rather than Engineering) onto "upgraded" versions of an item. too, a 'new generation' device doesn't automatically mean it's "better" than the previous version - many cases of newer-model items to actually be less-capable than previous generations, regardless of what the specs say. if you feel like doing some poking, try some of these (I'm assuming that both your sticks were tested on the same TV, not different TVs):

1. borrow another same-model stick from someone and see if the same behavior occurs
2. make sure your stick is running whatever the latest firmware/OS is provided (not sure if Amazon does updates)