Well, my own theory is that Compiz still needs the nvidia driver, which
is required to be unloaded manually when the system enters suspend as
the driver doesn't respond properly to the suspend request from the kernel.

Once the system is restarted the nvidia driver is reloaded manually.

When this happens, since Compiz is in use when the system enters suspend
and thus its full state and connection to the nvidia driver is stored
in memory (and thus resumed where it left off) compiz has no idea it
needs to re-establish a link to the driver (since it has been completely
unloaded and reloaded).

Ideally, a script would unload compiz and reload it after the driver in
the same way, but not having been a programmer in some time I am not
sure I can write a driver to do that.