Friction seemingly occasionally disappears in PhysX 3.4

I am seeing a strange bug where occasionally friction seems to disappear. See the short clip below where boxes will occasionally slide over each other, despite the friction coefficients being set to 0.3:

In particular, pay attention to the behaviour at the beginning and end of the above clip.

I have tried playing with the contact offset and rest offset properties. The video shows the contact offset set to 0.0015 and the rest offset set to 0.001 for all collision shapes. With these settings I have found that the sliding occurs more frequently. However, with these properties set to their default settings I still observe sliding, albeit less frequently.

There’s nothing unusual about this particular scene. The boxes have masses of 1kg and dimensions 0.4 x 0.4 x 0.4m. They are each set to use 8 positional solver iterations and 2 velocity solver iterations. Increasing the number of solver iterations made no difference. The friction type is set to eTWO_DIRECTIONAL. Setting eENABLE_STABILIZATION made no difference. The PhysX processor is set to CPU. I am using the latest release of 3.4 (including the latest patch).