Ahh, I c what you did there.
I just read through my N64 dev docs and found out, that it seems to be a problem with the RSP memory registers, to be specific the SP Registers.
According to my docs:
SP_MEM_ADDR_REG (0xA4040000) - Location to DMEM or IMEM to write to, commonly zero.
Bit 12 selects (1) IMEM or (0) DMEM. So to transfer to/from starting at 0 write 0x1000 for IMEM or 0x0 for DMEM to this register.
Nemu64 states, that PD 1.1 in 4MB Ram mode writes an invalid value to this place.
The problem is, that nemu64 has a non changeable rsp directly included into the emulator, so it seems that there's no way to fix it sadly.
The source of nemu64 0.8 sadly was never released only the gfx plugin had a source release (located
here), but that won't help you... sorry.