Stock agencies reject artworks if they have locked items in their opacity masks.
It means that these agencies probably have automated solutions to check for issues like that... meanwhile, Illustrator, the leading software to create vector artwork, has none.
The workaround would be to check each potential object manually, or use scripts that still have no chance to fully check the problem because of certain limitations.

Basically we need a strong Preflight tools. There are some (like Vector Inspector Pro), but a native one could be faster and more efficient, if done correctly.