X-rays were negative. Brewers also got him x-rays when the WBC ended and he returned to them, negative again. It wasn't until later after experiencing discomfort after a check swing that an MRI revealed a hairline fracture at the base of his middle finger. So it's a very tiny fracture that didn't show up on initial images.
https://www.mlb.com/news/jackson-chourio-has-left-hand-fracture
"Chourio was hit by a pitch on March 4 while playing an exhibition game for Venezuela in West Palm Beach, Fla., ahead of the World Baseball Classic. X-rays at the time were negative and he sat out the first two games of the tournament -- but he had been playing ever since, including both of the Brewers’ exhibition games against the Reds in Milwaukee on Monday and Tuesday.
On a check swing during one of those games, Chourio felt renewed discomfort and went for an MRI scan, which revealed a small hairline fracture at the base of his middle finger."
Murphy said. “Hit by a pitch doesn’t mean you’re out. He said he felt fine. They scanned it there, no fracture. Came back [following the WBC] and they scanned it, no fracture. Then it kept giving him problems so they went in and did an MRI and can see things other scans can’t see.