When DMA from ROM with fixed address mode, with an address adjacent to 0x20000 boundary (i.e. 0x0801FFFC in 32 bit mode) non-sequential timing is used for all accesses.
For example, a DMA of length 32 will use 32 non-sequential accesses when reading from ROM.
When DMA from ROM with fixed address mode, with an address adjacent to 0x20000 boundary (i.e. 0x0801FFFC in 32 bit mode) non-sequential timing is used for all accesses.
For example, a DMA of length 32 will use 32 non-sequential accesses when reading from ROM.
This is demonstrated here: https://github.com/alyosha-tas/gba-tests/blob/master/DMA/DMA_ROM_Fixed.gba.
This is also true in decrementing mode, which is required to pass your 128kb-boundary test rom.