I would have assumed it was dropped because it tastes grosssssssss

I’m not sure if I buy the claim that it was due to Gothic influence. At the end of the day, seasoning typically falls into disuse because of either supply issues (see: cumin, garum) or because it’s superseded by another seasoning (see: long pepper, superseded by hot peppers). Frankly I’d expect either to be the case with fresh coriander, and then the decrease on the consumption of the fresh stuff leading to lower availability of the seeds.