Noticed that the last submit to src/video/ps2sdk/SDL_ps2video.c had the following line added:
+ gsGlobal->Test->ZTE = 0;
This is not 100% OK when you want to disable ztesting because of a bug in the GS which could cause issues on atleast some PS2-revisions. Instead, keep ZTE as 1 and instead set ZTST to 1 (pass all pixels), which is equivalent.