When Vengeance and Arm's Length are both active, Arm's Length will never trigger.

Date & Time: First discovered while levelling WAR during early access, rediscovered 2019-07-18, 2305 PST
Frequency: Every time
World name: Excalibur
Character name: Kestrael Sthalsyngwyn
Class/Level: Warrior/80
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 Vengeance, then use Arm's Length before Vengeance wears off.
6. Note how no enemies are ever afflicted with Slow.

Honestly, not surprising this was never caught sooner, until this expansion, WAR was the only tank with a skill with a return effect, just another one of those spaghetti code things, I guess.

Have received independent confirmation from FC mates that this does happen for them as well.