Why is this happening?
It's very simple. The video is not being decoded by this player. It's that simple.
Why people expected more is that their PC, Apple or other thing would play it yet there is no PC in that NAS or Sony device so the decoding that is there is limited.
Many other discussions but let's keep it simple. Fixing this is too much for some. It involves figuring out what formats or encoding is supported then converting to that format.
Many say forget this and use an Apple or PC.