Just a heads up for anyone who is curious, I managed to find a workaround for the issue that caused the last enemy killed to sometimes not play its death animation before the battle ended/victory dance ensued.
At first I thought it was a bug I'd have to live with, but I made a custom animation. Once I finished I realized my new skill didn't have the issue. The last enemy killed, when killed with that move, played its dying animation.
It seems it is related to both the skill's sequence as defined in the LeTBS config AND the actual skill animation in RPGmaker.
My move (working), I realized had a 25 frame animation and spark (broken) had a 15 frame animation. I added 10 more frames to Spark. Still not working. My move was also using the default skill sequence whereas Spark had a custom one. So I set spark back to 15 frames and told it to use the default sequence instead. Still broken. So I turned it back up to 25 frames. Now it works.
I'm gonna go through all of the built in sequences and the RPG maker animations the skills call to see if I can adjust them to allow enemies to actually die when they're the last target.
Not sure if anyone cares but me, but hopefully that'll help someone else who wants to use this.
TL;DR, the last enemy doesn't die because the attack sequence is goofy and animations are too short, and as a result battle concludes before it has played its death animation. If you make them longer the enemy will probably play its dying animation.
Okay, figured it out:
The sequence for default skill included this line:
"effects: {aoe}_battlers, current_obj, obj_anim, 0, true",
While default atk had this:
"effects: {aoe}_battlers, current_obj, obj_anim",
I found that adding the 0, true and increasing the number of frames in the RPGmaker attack animation to like 20 or so fixed the problem for default attack. Same for Flash Jump.
Anyone know what the "0, true" signify?