I've made a similar functionality in the past; I've made a ticket overview on which the user could hover on some columns in order to show a more elaborate description of the ticket. The description wasn't included in any of the data grid columns so in my case I didn't use the attribute I displayed on the tooltip page and this worked fine.
I cannot vouch for newer Mx versions, but I've just checked my app and it's currently is in version 7.5.0 so up untill that version I suspect you shouldn't have any problems.
Did you enable the 'Show tooltip' option on other columns as well (besides the attribute you're referring to in your post)?
I haven't run into this issue before. I'm in 7.15.1 right now so it might be a different case but I have a datagrid that has a tool tip page that shows a list of attributes that are not shown in the datagrid, as well as a listview over an association. I haven't run into an issue yet where certain fields were not showing up in the tool tip page.