When typing in card mode on a table, and filling in a text field, the entry is lost if it contains a newline (using a Mac, at least). That is, whatever you typed will not be saved to the underlying sheet; it vanishes. Along the same lines, when copying multiple paragraphs into a textfield, in card mode, only the first is copied (when it gets to the line break it stops copying). However, I can copy into a text editor so the entire text block is in the copy buffer: it just isn’t accepted as input into the Coda textfield.
If, however, you first type one line without a line break (generated with option-return), then come back, you can type all the lines you want. If you type multiple-lines directly into the grid, not in card mode, multiple lines are handled fine.
This bug is reproducible for me. Using macOS 10.13.4 w/ Chrome 66.0.3359.181.