Here is how to reproduce:

1) open a new writer document
2) press enter to produce some newlines
3) type "ae" at the last line
4) mark the text "a" and insert a new bookmark (Insert->Bookmark) called "A"
5) place the cursor between "a" and "e". Please ensure that the cursor is
collapsed and doesn't mark any content now.
6) type "ff"
7) use the navigator (F5) to select the bookmark A and see that the text "a" is
selected. So far, everything is fine and the behaviour is correct! Now lets come
to the bug:
8) place the cursor between "a" and "f" (collapsed) and press enter twice.
9) select the bookmark A via the navigator. Now you can see, that the bookmark A
extends over 3 lines which should not be. I would expect the bookmark to still
contain the content "a" exclusive (and not the newlines also).