Hm that’s odd, I wonder if your TV dislikes the very large overscan area that results in the PSP optimized mode.
Basically, the way that it works is that the OSSC tells your TV that the letterboxed area is in the overscan area, so your TV is the one that needs to drop the “offscreen” pixels. Unlike most pictures though, the “offscreen” area of this image is very large because it includes the letterbox.
What happens if you disable the PSP sampling mode, but keep Line2x turned on (make sure upsample2x is on)? If that works correctly, then it’s not a pixel clock issue and instead is probably caused by the large amount of overscan.
Another thing to try would be testing it on another TV. It works well on my device, but if there’s TVs out there that dislike the overscan region, that will be news to me.