Yep, I see the same thing:
The symbols I've highlighted are clearly italic but they should not be.
You can always use the actual Unicode characters within quotes, as I did in the bottom row. At least I don't see any problem with doing that, and it's less typing.
Unless someone has an explanation as to why "nitalic" is not working here, I think it needs to be filed as a bug.
PS: There's one more approach you could use, but it's a fair amount of work to set up. Define your own symbols, e.g. something like %vOMEGA ("vector OMEGA"). You can define the exact font and character should appear for that symbol, so you can make it bold, non-italic, whatever you need.
You define the symbols using Tools > Catalog.
It's a lot of work to define the symbols, but once you do, it's easier to input them in a formula.
AOO4/LO5 • Linux • Fedora 23