Most world grain is shipped short distances within countries (by rail, truck, canal, ..) and consumed locally. Only a small proportion is shipped internationally, and usually the shortest possible routes are picked (because that is cheaper).

Entirely taking out Ukrainian grain shipments is not a small sliver of the total amount of long-distance grain shipping, but instead requires a significant increase in the total deadweight-ton-miles. We are talking about taking 10%+ of international wheat/corn/barley trade and making it travel 3–6x further than it otherwise would, in an industry that usually runs on thin capacity margins. (Disclaimer: I am not an expert and have not done a detailed calculation.)

That is a potential logistical nightmare. Hopefully smart people with the power to make large temporary changes are working on ameliorating the pain and keeping everyone fed.