Dawntrail edition. Bug continues to exist five years and three expansions in. While it is true that no skilled WAR would use the two in conjunction with each other, instead seeking to maximize outgoing damage output via Vengeance/Damnation's thorns effect, it is, regardless, an unexpected interaction.

Date & Time: First discovered while levelling WAR during Shadowbringers early access, rediscovered 2019-07-18, 2305 PST
Re-rediscovered 2021-12-05 0127 PST
Re-re-rediscovered 2024-07-07 0200 PST

Frequency: Every time
World name: Excalibur
Character name: Kestrael Sthalsyngwyn
Class/Level: Warrior/100
Party or solo: Both

Steps:
1. Pull many enemies.
2. Stop, use Arm's Length to verify that they are not immune to Slow.
3. De-aggro, wait for Arm's Length cooldown.
4. Pull many enemies.
5. Use Damnation, then use Arm's Length before Vengeance wears off.
6. Note how no enemies are ever afflicted with Slow, despite being hit multiple times.

Both Vengeance and Damnation are affected by the same interaction. As noted, this is the third time the bug is being reported, having first been discovered in Shadowbringers. Two bug reports previously have been posted and accepted:

https://forum.square-enix.com/ffxiv/...er-trigger.-EW
https://forum.square-enix.com/ffxiv/...-never-trigger.