I don't think it's safe to generalize from the limited number of chisels that you inspected to entire classes of steel.
The voids you saw in the Ashley Iles chisels are probably where carbides fell out of the ferrite matrix. AI uses HCS, so they *shoudn't* have that problem, but there are all sorts of ways get coarse microstructure via botched processing. I'm honestly not impressed by the modern English edge-tool makers, and not altogether surprised that your chisels have that problem.
More broadly HCS (new or old) and Hitachi White are more similar than different in terms of composition and achievable grain structure. When you measure "differences" between them all you're really doing is measuring the quality of the processing that was applied to specific tools IMO. I'll go ahead and stipulate here that the best Japanese chisels receive exquisite processing.
Even if AI's HCS were optimally processed you can't generalize from that to "modern processed steels" the way you did, because the larger category includes exotics like PM steels that mop the floor with their older counterparts in all objective respects save cost. You can't really generalize unless/until you've looked at those comprehensively.
In short, your data entitle you to draw conclusions about a couple/few specific tools, and *maybe* some weak conclusions about their makers (though you'd ideally want a much larger sample for that). You certainly are not on solid ground when you generalize to entire eras/categories.
Really the only thing you can safely and objectively say about modern vs older steels is that the modern ones have higher background radiation (due to atmospheric nuclear weapons testing during the Cold War).
Also, if you want to see some serious voids have a look at the D2 Ray Iles pigstickers. He actually does a good job with the processing, but there's only so much you can do to refine the grain structure of a conventional steel with 12% Cr.