Up/down particle directions are reversed

Partner Rank:

3

Partner Team Reporting:

Everbloom Games

Verification builds:

1.16.200.02

Summary:

Particles with a directional facing_camera_mode render in the wrong direction when facing straight up or down.

Impact:

In an upcoming map of ours, we use particles that face straight up or down quite often, and this bug makes them invisible from the intended direction, making players not know when they're about to fail, or ruining an illusion. We can technically make them face the opposite direction, but this will result in the same negative effect on players in future versions where this bug is fixed.

Repro Steps:

1. Open the provided world
2. Head to the magenta-floored chunk
3. Look at the two rotating glass blocks with gold planes inside. These have rotating directional particles facing the opposite direction from each other. 

Expected:

The planes don't flicker when facing up or down (aside from the texture flipping)

Observed:

When a plane is about to reach its up/down position, it switches directions for a few frames, making it invisible.

Screenshots/Videos attached: Yes/No

Yes.

Regression Builds:

Unknown

Notes:

Gfycat