I used Olvia last weekend for a flight from the UK to Belgium without any problems
This is v. interesting, but there are counter examples reported where such flight plans disappeared.
It is possible that "Belgium is near France" so they let it go. A bit like Homebriefing, based in Vienna, do unlimited free flight plans to/from not only Austria but also Switzerland.
Until somebody manages to get an official clarification, the situation will be undefined.