Back before I became a triple-mode BN, I wals thought it was my duty as a BN to ensure that the other modes were checked by someone who knew what they were doing, even if they didn't give an official nomination. And as a triple-mode BN, there were plenty of times where even though my main mode was standard, I only checked the ctb / taiko diffs without looking at the standard ones, because I was not responsible for those for that particular case.
I think depending on the issue that the map got disqualified for, the responsibility may or may not lie on the nominators. For example, in a Standard/Taiko/Ctb set, if the map got disqualified for an unused hitsound, or something general, obviously the nominators involved are responsible. You should check that regardless of mode. But if the Mp was nominated by a Standard and Catch BN, but dq'ed for a Taiko-specific reason, I feel like the Standard and Catch BN's should not be punished for that. However, With regards to the nomination process, I think it would be a requirement then, that the map be nominated by a Taiko BN to confirm officially that that mode is good.
I believe that would resolve the issue of BN's being either held responsible or not held responsible, as well as how the map can proceed with requalification.
Caveat:
I don't know about the community now, but it used to be very easy to get quick opinions from BN's of other modes when checking a hybrid set. I don't know if that is still reflected today, since I'm no longer familiar with the BN ecosystem. I used to (and I guess still do) host a lot of hybrid sets, and nominated many of them too. As i see it if you didn't get additional opinions on a hybrid set before nominating, it was clearly your fault. Like, for me it was always an expectation to get opinions. Maybe nowadays people just don't care. If that is the case, perhaps something should change in the BN community instead to make mode-specific opinions like this easier to access, or you should adequately punish BN's for not caring so they'll care more. (Though I feel like nowadays QAT's have completely overdone it, and BN's are too scared for the wrong reasons lol)