War machine slide with only one WM

War Machine slide with only one WM is a rare glitch in the Underground level where the second (or perhaps first) War Machine doesn't initiate the slide chase thus breaking the scripted sequence.

General information
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How and where to get it
This glitch can occur in the last section of the Underground level and requires specific steps to set up (details are still being investigated).

It is possible to get no WMs or just one WM patrolling the slide before you dive in; there are also triggers on the slide itself that may summon one or both WMs to chase the wayfarer (exactly how and why is not clear). One way to trigger this behavior is by going out-of-bounds around the level up to the green mountain/plateau on either side of Patrol Halls, and jumping into the last section of the Unfriendly Tunnels (right before the slide, thus skipping all normal WM spawn triggers).

Another trick that appears to impact the slide especially when playing with a companion, is to launch into the slide from the top of the nearby statue (west of the gate) aiming upwards, this seems to break one of the WM triggers. If you complete the slide but your companion doesn't, you can then walk back up the slide and there will be a WM patrolling the area; in my case (qtzl3000, July 2020 on PS4) WM was patrolling with the light on and got me even though I've already completed my slide.

Gallery
The video below shows the only recorded evidence of this glitch: