I had set 0.10 below paragraph spacing for some reason; if I set it to 0 it seems OK.
If I set it back 0.10 the unevenness recurs.
Clicking Print Preview clears the unevenness.
I might change to latest version as my test version. First I see there is a Windows update I can download.
Version: 7.5.3.2 (X86_64) / LibreOffice Community
Build ID: 9f56dff12ba03b9acd7730a5a481eea045e468f3
CPU threads: 8; OS: Windows 10.0 Build 22621; UI render: Skia/Raster; VCL: win
Locale: en-NZ (en_NZ); UI: en-GB
Calc: CL threaded
[Edit]
After Windows update and restart, still same. Installed 7.5.5.1 parallel still the same. I tested on another Windows 11 computer (LO 7.3.6.2) and same thing so your guess that it is a Windows thing is probably correct.
Toggling Formatting Marks solves the problem.