This is also a compabiltity issue to MS Office:
Line style and thickness cannot be selected separately.
There are two dotted line styles available, but MS Office 2003 has 5 styles
and the line thickness can be chosen separately.
The converted Word documents would look more similar, if the line styles to be used are more similar. Herefore it is also necessary to make a separation between
style selection and thickness selection.
Taking it as I'm already working on it. See progress on the git repos here:
*** Bug 34440 has been marked as a duplicate of this bug. ***
Borders work has been integrated in master. This is fixed in future 3.4.