Created attachment 172821 [details]
Textbox with fat border
Open attached document. It has a text box with fat border and is positioned with "Alignment relative to Margin" horizontally and vertically.
In the lower part of the page is a screenshot how it look in Word.
1) Word aligns the outer edge of the border, LibreOffice the snap-rectangle, to the edge of the page text area.
2) Word measures the text padding from the inner edge of the border, LibreOffice from the snap-rectangle.
Created attachment 172822 [details]
Screenshot Word - LibreOffice side by side
I confirm it with
Version: 18.104.22.168 (x64) / LibreOffice Community
Build ID: a529a4fab45b75fefc5b6226684193eb000654f6
CPU threads: 4; OS: Windows 10.0 Build 19043; UI render: Skia/Raster; VCL: win
Locale: de-DE (de_DE); UI: en-GB
in comparison with Word 2016