I figures this would be solved after beta 1. It took a little longer to go to the flashing question mark but its still happening.
Same happens on my iMac 2017. If I leave it sleeping for a prolonged period of time it will end up with that flashing mark sign and unable to awake. Short sleep works fine.
Experiencing it on Beta 2 and 3. I tried checking drive with disk utility from the recovery mode (everything's fine), resetting NVRAM and then setting startup disk (have to set the boot drive after reset, which is a known issue in this beta).