The OOXML spec allows non-printable characters to be encoded using an escape sequence _xHHHH_ where HHHH is the hex code of the character.

I have received a few xlsx files that contain _x000D_ sequences (the carriage return character).

If you load the file in creek (or open the xml file in a text editor), the cell contents will be
"first line_x000D_\nsecond line"
Where _x000D_ is the carriage return character, and the line feed character is encoded normally.

When you open it in excel you just see a newline.

see:

• New line in cell can create _x000D_ roo-rb/roo#223
• New line in cell can create _x000D_ tidyverse/readxl#51