If you are customizing the GUID value of a post in WordPress, make sure you maintain the format of the GUID as a valid URL format. There are security checks on the value that enforce the URL format.
If you pass in a UUID as the GUID value when creating a post, your value will be discarded and the standard auto-generated GUID will be used. If you try to force in your UUID afterwards (say, with a direct SQL update), your GUID will be turned into an empty string the next time you edit the post via the admin.
Using your own GUID format is easy and well supported, as long as it’s a valid URL.