When copying and pasting content inside tables, in Confluence, it behaves different depending on how you insert the text. Here's the steps to reproduce and better understand:

With Shift+Enter together:

1. Edit a page and create a table
2. Enter text separated by lines (using SHIFT+Enter)
3. Copy the text and Paste in other table:
   

With Only Enter

1. Edit a page and create a table
2. Enter the text separated by lines (only using ENTER)
3. Copy the text and paste in other table
   

Screenshot attached for further reference.