Among hitters with 200 PA Hiura is 2nd in HR/FB and one of only two players with a HR/FB rate over 30%. Here are the players with 10 highest HR/FB rates in baseball and their Max EV percentile.
Name - HR/FB rate, Max EV percentile
Aaron Judge - 36.2%, 99th
Keston Hiura - 35.0%, 69th
Oneil Cruz - 29.3%, 100th
William Contreras - 29.2%, 97th
Giancarlo Stanton - 28.9%, 100th
Yordan Alvarez - 27.6%, 97th
Michael Harris II - 26.5%, 88th
Trayce Thompson (lol) 26.2%, 80th
Byron Buxton - 24.3%, 92nd
Mike Trout - 23.6%, 95th
Hiura has always been on the higher side of HR/FB rate with a 23.9% career average but he's upped that level this year to Judge level despite having a 69th percentile Max EV. I think it is safe to say that Hiura probably couldn't maintain such a prodigious HR/FB rate with everyday playing time considering his physical limitations.
If Hiura's HR/FB rate was his career average he would have 10 HR instead of 14. If you give him 2 doubles and 2 outs instead of those 4 HR his OPS would drop from .780 to .722. People talk about his amazing numbers against RHP but he is rocking a 44% HR/FB rate against RHP. The next closest player with 100 PA is Judge at 35.4%. I'm guessing the Brewers just don't think those numbers are sustainable for Hiura over extended playing time.