They messed up. Everyone knows that hydrogen is a small molecule and sneaks out through the smallest hole. Should be even worse in space, with zero external air pressure.
The core stage contains reused space shuttle engines, which are hydrogen fueled. That has some benefits, such as high specific impulse and is clean burning, but it costs a ton of money to pump all that supercooled liquid hydrogen in just prior to launch. One wonders why they couldn’t just design a newer engine, or why whatever bugs they are encountering weren’t cleared up in the 20 or so years that the space shuttles were operating.
How did they do it in their heyday, I wonder? Can’t they use the same type of seals they used back then? They are copying everything else…