Trag, I have zero doubts about it. It is truly a ROM code issue. The issue could be fixed by someone who knows how to edit the ROM, but I am not that man. Indeed, I doubt anyone other than an X-Daystar engineer could do the job right.
I've also tried two different boards and 3 different ROM versions (I had the ROM updated in one of my boards to the latest version -- a guy here in Japan does that for a fee). None of those resulted in a working 1.4MB floppy when a non-stock ROM was in there. I mention this only to say that no Daystar ROM that I know of will make the Turbo 040 properly handle 1.4MB floppies when used along side a non-stock ROM. Hence the need to fix the ROM code.
I've also tried two different boards and 3 different ROM versions (I had the ROM updated in one of my boards to the latest version -- a guy here in Japan does that for a fee). None of those resulted in a working 1.4MB floppy when a non-stock ROM was in there. I mention this only to say that no Daystar ROM that I know of will make the Turbo 040 properly handle 1.4MB floppies when used along side a non-stock ROM. Hence the need to fix the ROM code.