Cell border adjustments not visible in document but still appearing in "preview" (mac PDF viewer) and when printed

Am using LO

Just as the title says.
Also I find it tends to pick and choose- for example adjustments to diagonal lines within a cell, such as an increase in width seem to show up in the document, whilst adjustments to outer borders of a cell, such as an increase in thickness and change of colour, will only show the change of colour but not the increase in thickness.
However, once printed the document will look as it should- in this case with an increase in border width as well as a change in border colour.

Makes it rather difficult to keep track of the changes I’ve made regarding cell borders- currently I have to select “print”(which will display the document in “preview” (which is Macintosh’s default PDF viewer)) just to observe any changes I’ve made, as the changes show up just fine in “preview”, but not in the document itself.