To reproduce:
1. Create Mixed Reality (attach the third Vive controller or use a virtual one);
2. Open attached project;
3. Open "ReproScene";
4. Choose the "New Post-Processing Profile" in the assets and choose "Bloom" effect in the Inspector;
5. Enter Play mode;
6. Look at the Top Right quadrant in the Game View.

Expected result: Top right quadrant gets a correct alpha mask.
Actual result: the Top Right quadrant is rendered white when using PostProcessingBehavior in Mixed Reality.

Note: this happens with following effects:
1) Depth of Field;
2) Motion Blur;
3) Eye Adaptation;
4) Bloom;
5) Color Grading;
6) Chromatic Aberration;
7) Grain;
8) Vignette.

Reproduced on versions: 5.5.2p3, 5.6.0b3, 5.6.0f1.