This happens when using Stalker's Mark on a Guardian protecting the base.
When the Drifter tries to teleport behind a Guardian using Stalker's Mark's secondary action, there is a high chance that he will end up beneath the floor. Sometimes it doesn't happen, not sure what decides that.
Below the Hidden King's base there is a void, but it's possible to stand right underneath a single pair of Guardians. If the Drifter falls into the void he will be teleported to ground level.
Below the Archmother's base there is solid ground, so it's possible to walk from one pair of Guardians to the next. The Drifter can leave by "unsticking" through the menu while standing close to a wall.
Here's my own session from today.
I was able to exploit the Guardians on both the left and right sides of the Hidden King's base.
Game ID — 62285779
Guardians: York — 17:00
Guardians: Greenwich — 19:45
In my testing I've been almost completely invulnerable to all damage from above the floor, though the Guardian's melee attack (if triggered by our troopers) did push me aside, which I find fascinating.
Eventually a Guardian did seem to be able to attack me, but it had to directly target me to do so.
