When copying a multiline text inside a Cell in Calc, in edit mode, if the text is available in the Clipboard as Rich TExt Format and it contains soft newlines (Shift-Enter), then the newlines are correctly shown in Calc. but after closing and reopening they are lost, and the lines are all joined together (nor even a blank/space to separate them).
See steps for detail.
It might not a big deal that "importing" loses some formatting. But 1) this is more than formatting, 2) it looks very bad and unsettling that something looks ok when working with the document, and when you save and reopen you get something radically different.
Steps to Reproduce:
1. Open a new Calc Spreadsheet . Enlarge the first cell, say to half the window width and height.
2. Open Windows Wordpad (or some editor that copies in RichTextFormat) (*)
Type several lines separated not by hard end-of-paragraph newlines (Enter) but instead by soft line-breaks (Shift+Enter) (**)
3. Select those lines, copy to clipboard (Ctrl-C)
4. In Calc, focus in the big cell in edit mode (F2), and paste from the clipboard (Ctrl-V)
5. All should look basically ok (also if we switch to another cell, sheet, document, etc). Close Calc, saving the file.
6. Open the saved document.
[*] I experienced this with a programmers text editor (EditPadPro) that, as a bonus, copies the text also as rich text format.
[**] these in RTF correspond to \line commands
Result: all newlines are gone. They are not even replaced by spaces, the original lines are joined together (a mess).
Lines should be lines. As were shown originally when copied inside Calc.
User Profile Reset: No
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.84 Safari/537.36
I reproduced (using Writer to produce the text), but I found bug 89920 which is essentially about the same root cause. I confirmed saving to XLSX preserves the line breaks.
Arch Linux 64-bit
Build ID: c3764c6848bd5ce0bbea2a82bedc3f0d55f01dce
CPU threads: 8; OS: Linux 4.14; UI render: default; VCL: kde4;
Locale: fi-FI (fi_FI.UTF-8); Calc: group threaded
Built on December 19th 2017
*** This bug has been marked as a duplicate of bug 89920 ***
If I execute the macro:
my_cell = ThisComponent.Sheets(0).getCellByPosition(0,0)
my_cell.String = "aa bb" + chr(10) + "cc dd"
Then I end up in  where simpy the AdjustRowHeight is missing. Imho, bug 114398 is not a duplicate of this one, since it ends in  where no string or edit cell is set.
I set it to NEW then.
I think this is probably a duplicate of bug 103829, and thus should be fixed in LO 7.5 (where text:line-break is now both imported and exported again).
(In reply to Justin L from comment #3)
> I think this is probably a duplicate of bug 103829, and thus should be fixed
> in LO 7.5 (where text:line-break is now both imported and exported again).
Yes, it is fixed, thanks for notifying.
Version: 184.108.40.206.alpha0+ (x64) / LibreOffice Community
Build ID: 00c5b0ca9264c5440bc70a68c425127ba5a47003
CPU threads: 2; OS: Windows 10.0 Build 19044; UI render: default; VCL: win
Locale: fi-FI (fi_FI); UI: en-US
Calc: threaded Jumbo
*** This bug has been marked as a duplicate of bug 103829 ***