GLX direct rendering should not be blocked by Grabserver when the client is made impervious to server grabs by calling XTestGrabControl.
When using the NVidia driver, it is blocked however.
When using the Nouveau driver, it is not blocked.
Therefore I think that this bug is in the NVidia driver or NVidia client libraries.
The details and instructions for reproducing the bug are described in Issue #878 for xorg/xserver at freedesktop.org: