How are you populating the value on the custom form? Is it bound to an external vRO workflow/action?
As far as the custom form goes it's just another custom property. There is a vRO workflow that as I mentioned, deletes the property, and recreates it daily. I could be doing this manually with the same result.
The trouble is that you have configured your property definition with a static list. If you inspect the custom form element and click "Values" you will see the static list. Deleting and recreating the property won't update that list on the custom form.
What you should be doing is binding your custom property to a vRO action that returns the values. It either queries SNOW dynamically or if that's too slow/only necessary once per day, then your current process should write the values somewhere local (e.g., resource element or a key-value store if you have one). The action bound to the custom property then queries that local source and returns the array of strings.
This way loading any form (default or custom form) will always pull the latest information.