To replicate, i LO Writer, have a single sheet document with lots of paragraphs in it (blank or otherwise).
Insert a footnote into the page:  "Insert > footnote/endnote".
Click "OK" in the next dialogue box, accepting the default settings. Scroll down to have the footnote in view.
Now edit the footnote, right-click in the page and then select "Page" from the context menu.
Select the 'Footnote' tab of the dialogue.
There are two sections to this page; 'Footnote area' and 'Separator line'.

To demonstrate the behaviour, change either, or both, the 'Space to text' dimension in the 'Footnote area' section, and/or the 'Spacing to footnote contents' dimension in the 'Separator line' section.

Select "Apply". As expected, the changes are executed on screen and the dialogue box stays open.
But ...
The dimensions in the dialogue box now change.
1.    If you click "OK" to exit, these newer altered dimensions are applied.
2.    You cannot, having clicked "Apply", change the dimensions to anything meaningful.
3.    You can exit, having pressed "Apply", by clicking "Cancel" and having the changes which occurred on "Apply" remain in place.
4.    Having pressed "Apply", you can then press "Reset", re-do the settings, and then click "OK" to exit.
If "OK" is clicked without "Apply" at the outset, the results wanted are obtained.

Result obtained on WinXP Home (Service Pack 3)
LibreOffice 3.4.5 000340m1 (Build 502)


A response on the LO Documentation Team mailing list to this was "I get the same thing on windows 7 as well as ubuntu 11.1--both running version 3.4.4 build 402"