Hi there! I think you have screen burn in - I know you already sent your unit out, but this is to help anyone else who might have had this.
I don't think the firmware update did this. You likely left it at -58 volume, USB input, for a while and that's why it looks fine. In your second picture, in the "M" for Mute, you can actually see the "-" part of "-58".
i.e. with the new firmware, if you took a picture at -58, USB input, PCM, 44 kHz, it would also *likely* look perfectly fine.
This exact same thing happened to me. I left mine at -32 for a long time. So -32, USB, 44 kHz looks perfect, but if I change the volume to anything else, or run a different sample rate, or change inputs, the uneven display lighting happens.
I love ifi. I have a whole boatload of their products and *ahem* sold a few units by recommending them to my friends.
But this whole screen burn in thing is a bummer. I wish they used a different screen, or have the screen just turn off automatically (as the default)...or something. I know I could have sent it away for repair, but I just decided to sort of live with it.
Lovely sounding piece though!