FF2us 1.0 doesn't have absorb set, so if it behaves the same way in that then the flag can't be the cause, as much sense as it makes.
FF2us 1.0 does have the absorb flag set. I just stepped through all the differences between the 1.0 and 1.1, and all but four changes are in the 18000-20000 range. I'm not sure why your version didn't have the Grudger absorbing lightning.
If you haven't figured this out yet, spirits have a built-in resistance to drain, I tested on my last playthrough.
That's not entirely correct. The Weeper monster (from the Antlion's Cave) is not resistant to the Drain/Blood weaponry. Only the absorb attribute is at play.
In relation to the discussion on whether or not spirits have resistance to Drain: I tested it, and for FF2us at least spirits are not resistant to Drain-elemental. I'm pretty sure this is the same for all versions. In reference to the Grudger: it isn't actually a spirit-type monster at all. It has no type (verified within the hex data).
Deathlike2 was right about the real reason the Grudger resists Drain-elemental weapons: it's because it has the drain/absorb flag set on their elemental defenses. The absorb flag only works for elementals lower than itself (fire/ice/lightning/dark/holy/spears&arrows). So the game is reading that flag as a "drain-resistant" defense. Two other spirit-type monsters (Spirit and Soul) have this flag set as well, which makes it seem like spirits are resistant. However, the Weeper is a spirit-type monster but has no absorption, which is why it isn't resistant to Drain-elemental weapons.
Here's some fun facts that I discovered while testing this. There are only five spirit monsters in the entire game: Weeper (16), Spirit (22), Hooligan (27), Soul (2E), and BladeMan (4E). Other enemies that you might think are spirit monsters (Grudger (3E), Screamer (4A), Ghost (54), Breath (80), Mind (83)) are not. The Grudger, Screamer, and Ghost are probably oversites, considering that they match the graphic template for the other spirit monsters. I always thought Breath and Mind were, but I guess that could have been just me. I'm not sure why the Hooligan is included . . . the other matching graphic template (Gargoyle) is a reptile, which seems more appropriate. The Screamer is actually classified as a machine and a giant, which I'm pretty sure is a mistake. Most of the other enemy types, if they had them, seemed to fit.