The only way this can happen is if there is a SmartApp associated with the lock. That’s easy to find out, click on your lock, click on SmartApps and see what’s listed. Remove all the apps and then see if it happens.
Are you sure that the ISY99 isn’t sending the lock command to the lock itself? If you’re using this device hander it will confirm if the AutoLock feature is disabled visually on the tile.
The other way to confirm, power off your ST hub. Then try it, now with the hub offline it relocks then it’s the lock or the ISY99. If it doesn’t relock then it’s likely an ST app.