Trail Effect = 100% gold drop?

My initial thought when I saw your video was that they could also be purple drops since the new mods also exist on them (blue too ? Not sure), definetly need more data samples as you said but great find nonetheless

I've only ever seen the new mods be generated on gold rarity items, but you can happily salvage and stick them on anything. The mods themselves are +4 or +5, both of which appear on golds.

I suspect this "you see it on them, they drop it" thing is common to quite a few enemies. Some of my observations:

• Star Sentinels that produce nonphysical hit sounds (their arrows sound like elemental wanding impacts) always seemed to drop a bow with the appropriate elemental mod on that matched the sound, though I didn't have a means to test the actual damage type on me. These are not necessarily gold rarity either.
• Star Tengu wielding celestial items seem guaranteed to drop them. It's like they don't exist in their pool of "normal" weapon skins that they can wield, at least in NM.
• Torment Claws that wand you (I did record a gif of this somewhere or other) seem guaranteed to drop the wand - though my sample size is tiny, the two in NM Veil I had wanding me with chaos projectiles both dropped Quicksilvers when I killed them with heroes flagged out of range.
• WiK Mantle that normally have strike as one (these usually hold scythes) can randomly equip bows, and seem quite likely to drop them when killed. Same deal with the Escape dagger bar. I've wondered the same about Peacekeeper Goons and shields. Same comments about not-always-gold-rarity also apply here. I imagine that non-gold drops can be eaten by loot scaling/high kill rate type effects, though.