Gaspare

You should use the Data Merge Layout Tool to set up a 2 x 7 matrix of cards on the page. That way you will define only the first card (as you already did, but it needs to be nested inside the layout object) and the software will replicate that for the other 13 cards. Still only one record will be previewed in all 14 cards on the page, but the generated output will use all records sequentially according to the rules you set in  the Data Merge Layout Tool.
The one record preview is one of the weird Affinity software design decisions which negatively impact usability.

There is no need to use a table. Here is an edited document: data merge new.afpub

YES also if I export only one page I see the error as seen in the two attached images