Certain scenes bad quality 4k stick

This is due to the NetFlix app on each device using different codecs for content encodings:

Your newer (RTD131x SoC-based) "4K Stick" NetFlix app is likely streaming content encoded with AV1 (or sometimes HEVC/H.265).

Your older (MStar C2 SoC-based) streambar/box NetFlix app is likely streaming content encoded with HEVC (H.265) or sometimes AVC (H.264).

So it all comes down to the NF app on the newer vs older devices streaming content using different codecs, and the encoding quality differs.

Unfortunately there is no way to configure the NF app/account to prefer one codec over another so you are limited to using different devices with different codec support to workaround encoding quality issues.

I suggest contacting NF about the AV1/HEVC encoding quality of The Walking Dead encodes - they will re-encode from time to time including due to customer feedback of a poor quality encoding.