Adobe Stock images disappearing from database

You can handle this only by licensing the assets as soon as possible. However, normally, assets should not disapear that often. There are several causes, that could make assets unavailable:

1.  The contributor decides to delete single assets, or even their account. This should be quite rare. Contributors will rarely delete successful assets. 
2. Adobe banns the contributor for violating the terms and conditions of the contributor agreement. In this case, the whole account is no more accessible. In some cases, after addressing the cause, the contributor is allowed to continue selling licenses of their content.
3. Adobe deletes some assets in a contributor's account. That may happen because the content is erronous, like six-finger-Joe with generative AI. A different cause may be that the contributor submitted duplicates of their content. In that case, you will have a chance to find an identical asset in the contributor's portfolio. 

The problem is that you can't know which of the reasons above lead to the deletation of the the asset from the database. However, with millions of assets in the database, this should not be a frequent issue.