If copy few cells, it is correct. But exceed small number will copy incomplete cells.
This is happened when using Chrome (Safari has the same issue, actually), other browsers won't happen such issue.

Reproduces:

1. use Chrome
2. open a sheet with some text data
3. select many cells had text
4. copy cells by Ctrl+c
5. paste cells' data to any text editor by Ctrl+v
   • only some cells will be copied