Styles don't preserve font sizes if the style uses 12 pt?

(Using OO 4.5.3.) My OO whole-document style is set to use 13 pt Helvetica as the (default) font. I defined a new named style that has just 2 differences from the default document style: the type face is different (Deja Vu Mono), and the font size is 12 pt.

When I create a new cell and then apply the style, the type face is changed, but the size is not — it remains 13 pt.

It appears that the problem is specifically due to the fact that I’m trying to use 12 pt in the named style. If I use a different size such as 11pt or 14pt, the size change is applied when I use the named style for styling text. If I use 12 pt, it’s not applied. This probably has something to do with some default somewhere, but I can’t figure out how to work around it.

What do I have to do to make a named style apply a 12 pt font size?