[stinkbeetle]

"working business trips"?

Have you read the requirements? Business visas or ESTA waivers have never allowed "work", there is nothing sudden about it. You can attend conferences and trade shows and have meetings. You can not "work" though.

I'm not an immigration lawyer so I don't know exactly what the requirements cover and what they don't. You are not allowed to "engage in active employment", but I have been permitted in paid for by my employer to attend meetings with company colleagues which is apparently okay.

I imagine a Korean engineer or project manager visiting to meet colleagues and inspect the site should be okay on a business visa or waiver. One who was there working on plans or overseeing construction might not be. You would hope the company had carefully checked these things.