Left eye goes black when using custom stencil buffer and post process materials using it

As the title says, whenever enabling the custom stencil pass and using a PPM with a stencil test, the left eye is black on my Valve Index and on 4.23.1. Another user was unable to reproduce with an Oculus on the same version.

The right eye seems to function properly (including the stencil test)

EDIT : After trying a bunch of stuff at random, changing the “blendable location” to “Before Tonemapping” fixed it. This happened after many different tests and engine restarts.

EDIT2 : Problem appeared again without any related modification. Changing blendable location and blendable priority sometimes intermittently fix the problem (after multiple tries), but only for a while.