I just installed Pixie on a new RG35XX H and I’m getting horizontal lines when playing Pokémon yellow on GBC.
I’ve tried disabling all overlays, shaders, filters and it’s still there. It’s not there in menus when closing the content. It seems to appear over time, and it’s not showing below a certain level of brightness.
This is what it looks like 
On discord there were hypothesis that this is due to anbernics shipping différent revisions of the display in that device and the timings being off for some. That’s a little above my pay grade but I’m including this info for context. See Discord
I own one of the affected devices so if something needs to be extracted from the stock OS install or whatever I’m happy to help however I can.
Did you find a fix? I have the same issue with my xx pro
Unfortunately, there is not a whole lot you can do… the panels are a bit of a mixed bag.
Because we’re essentially pushing display timings right up to the edge of ~60Hz, some panels just don’t like it. It’s not uncommon to see issues like scan line banding or flickering, depending on the specific unit.
For example, on my 35PRO, I get noticeable scan line banding at full brightness, but if I lower it to a mid-range level, everything smooths out. These kinds of quirks vary unit to unit. It’s essentially the silicon lottery at play.
Now, we could offer a fallback stock panel configuration that runs at 58Hz, which avoids some of these issues. However, that comes with its own downsides. At 58Hz, you’re much more likely to notice stuttering, especially with fast motion or horizontal scrolling content. People will end up saying “the screen is stuttering” even though it’s technically running as expected.
RetroArch can sort of fake a 60Hz output internally and mask some of that, but it only works within RetroArch itself. External emulators and ports will still show obvious stuttering and frame pacing issues.
So while it might seem like a fix, using a 58Hz panel profile typically results in a worse overall experience. Your best bet is to play around with brightness levels. It’s not perfect, but it can make a big difference depending on your specific unit.