eBay does not allow you to view Adult items without being logged in to an eBay account that has been age verified - and our gallery will not display Adult items outside of an Adult category to satisfy eBay's requirements regarding such items.
We are not aware of any issues that would cause the effect you have described with the gallery - on non Adult items the gallery should show with other non Adult items, however, as noted, if you have only one item that is not in the Adult category the gallery would not display at all if you shift your existing active listings over to the new active-content free version of the gallery. If you would like us to make this change for your account, please feel free to file a support case letting us know and we can remove the older version of the gallery on your active listings and append the new version of it to them. This should resolve the issue you refer to with the gallery on one item showing as Null.
If you continue to encounter difficulty with the scrolling gallery, please provide the specific details and an example listing or two in a support case so we can have a look into it for you.