But I am using a real Amiga and a real Amiga GFX card.
You are not using a "real Amiga GFX card". You are using a PC gfx card chip bolted onto your Amiga in a highly incompatible manner.
It can't directly do masked blits so it must emulate them. (that hack I gave u turns off the emulation).
It might also have broken vertical blank interrupts.
And of course it is missing 7 of the sprites.
And it has no copper and there is no way to track the raster beam. So it is completely alien and not Amigalike at all.
I thought GFX cards were meant to speed things up / make things better generally.
I thought they were to cause endless problems and make ppl get mad and hate the Amiga.
If you really ONLY use your Amiga for Octamed Sound Studio then you would have been much better off buying a hardware flickerfixer so you could run OMSS in 640x400 or 640x512 without flicker (that is how I do it)
I have a gfx card in my Amiga but I keep it disabled 99% of the time.