I have a table where the first row has 5 columns and on the following I did the merge to have only 2. On the desktop everything is ok on the mobile in the lines with the merge repeats the content 5 times. Sorry my english and thanks.
That is the default behaviour of responsive builder, you have to adjust items per cells value to get the desired layout. If you still have problems, please share a live url to your site where your table resides so we can investigate it more.
After getting some requests about this merged cell behavior, we decided to add a repeat header row option for responsive tables where top row as header is enabled to further customize that repeat functionality even more. It will be available with the next update (v.1.3.3).