mount_ps2_discfile
For PS2 Classics you can specify the media type in the
LIMG segment. (1 = DVD, 2 = CD)
For PS2 ISO, Cobra payload auto-detects the media type. By default it assumes the disc being mounted is a DVD. If the disc has more than one track or if CD001 is not found at offset 0x8000 of the ISO image or if the TOC size is 0x800, then Cobra assumes the media type is a CD.
I guess your ISO image could have a TOC size == 0x800. Maybe changing the TOC size at offsets 0x80A6 and 0x80AA in the ISO image could fix your issue. The TOC size is stored twice in little endian and big endian format.