We also just upgraded to Horizon 7.12 and are having the same issue running a postsync script. We are also not seeing the expected log file.
Instant Clone agent initialization state error (19): Failed to execute postsync script (waited 5 seconds)
Confirmed the postsync script is (still) on the master image. Guest Customization settings for the desktop pool have not changed. This was working without issue on Horizon 7.10.
