So, firstly, thanks to Lars for sending over the DQDX key - I still don't know what I did to get myself lost in some wayward corner, but I've now got both version of the game sitting right on my desktop. Which leads to... regression testing.
This "bug" might be easier to explain via video... but unfortunately my streaming/recordings skills are so bad I managed to make the videos silent by accident. It is probably still the easiest way to understand what is going on though is to watch at least one of the two "behavior" videos at https://www.twitch.tv/et11235813/videos/all
What is happening is that, in the course of mapping out speed run strats and other challenge modes (like 50% xp/scrap, only play each difficulty of each level once), I have a configuration for "Old Friends, New Enemies". In particular, the speed run version I've just tested has Ketta at level 4, Slak at L3, and two generics for both classes at L2 each. This can be achieved by hiring as early as possible for these two total, and perfecting every preceding level on normal. (Aka, what you'd do in a speed run.) I have a +4 bow and a +5 sword as the only weapons, and no armor.
The level setup puts the generic beserkers at the outside corners of the placable area, and puts each archer two back (right) from the beserkers. If you've seen Syl's speed runs, you'll note he avoids this setup with the weaker archer even though it should be the best damage configuration (he gets more weapons to cover) - and until now I didn't understand why. Boosting both archers will leave you unable to summon Slak until you kill some enemies - he goes on the top left center corner, covering the center lane (and loading up the weaker red lane up top via his corner bias).
Slak holding the only +5 sword, he gets boosted two 2 first, although other than some leakers it won't be too important to produce the bug. What happens is that, in DQ Legacy, the weaker top archer (archer2, or "WS" since that is the key command by default) takes a number of hits from the first ranged damage wave, but basically heals it all back. While a complete lack of evades might mean the last archer needs to be summoned to the right of "WS", most if not all of the time WS lives to the end of the level with major, but not fatal, damage.
On DQDX though, the second (stronger) wave of ranged enemies scores the kill on their way back after the long switchback. This is driven in no small part by WS lacking full HP going into that wave, because the regen healing is slower than the total damage that came in. This is before the third wave of ranged (which have more hp but not more damage), which would also score the kill if we could get that far.
So - at root... something even in simple early levels is different between the two versions, and it is killing archers in this particular strat in DQDX even as Legacy allows the strat. While it may be judged a bug in Legacy, it is hard to tell which is functioning to spec, or even what precisely the difference is between them. Worse, it may mangle the hero mode perfect balance, since those levels are very micro and super finicky. (Although this one seems to cut in Legacy's favor so far.)
Previously the Long outside run level seemed to have a lot more leaking worms on DQDX, implying lower damage - but there is a chance that is late timing at boosting someone due to higher speed in DQDX, or quite possibly just my bad memory.
Ninja edit note:
In the video I use frenzy and lightning to cover some leakers. I tried again more carefully and did not need to do this because I didn't run the game as fast. Even better, I figured out how to turn on sounds. Only problem was that I turned on sound at the expense of video... and hearing me talk about it with just a blank screen up is quite uninformative over all - I didn't bother posting that part of it. But while there are suspicions that there are damage output differences as well as the incoming damage issue, that is not confirmed - the apparent confirmation in the pair of videos may be due to slow reactions on my part.
(When I say "there are suspicions", it is the polite way of saying "I don't believe the new tech can mimic the old 'bugs' - where 'bug' here means a behavior that wasn't the intent as encoded in the config/data/animation files.)