As supposed (at least partly):
The horizontal tabulating the space behind the “line titles” ending with a colon inside the cell seems to be arranged misusing the character U+0001 which is one of the old control characters:
U+0001 1 001 [Start of Heading](https://en.wikipedia.org/wiki/C0_and_C1_control_codes) SOH / Ctrl-A
LibreOffice doesn’t support this kind of tabulating inside cells.
If it once did, it was a mistake mimicking MS nonsense “for compatibility” purposes.
I don’t know enough about xlsx to study the file structure.
The NewLine functionality inside cells is coded with U+000A as it also is done in Calc.
You may report a “bug” asking for the reinstatement of the older behavior.