Could be almost anything, but there is a lot that can't be fixed via firmware or microcode, if the underlying hardware isn't capable of it.
If, after seeing what's going on with Intel, for example, they wanted to cull the top of their voltage range, that couldn't be fixed without reducing stability or clock speed, which probably wouldn't be acceptable. Same thing if some samples had IODs that couldn't reliably do the 8000MT/s the new EXPO profiles are supporting, or needed more vSoC than they're comfortable with. Or they may be worried about performance comparisons and are culling the samples that don't meet revised V/F curves.
Better to sort this out, whatever this is, before bad reviews or failed parts cause worse issues.