I kinda don't trust that dictionary :/. Google fight gives 62 500k results for sub-area vs. 250k for subarea, which is usually a good indication ^.^. Also, my (non-native speaking, like me) instructor claimed it's sub-area. But the only dictionary entry I can find is that one, which has no hyphen...
"potential fields method". It's a method that is about potential fields, not a potential method that has to do with fields. Does that mean I should spell it "potential-fields method"?
I kinda don't trust that dictionary :/. Google fight gives 62 500k results for sub-area vs. 250k for subarea-- You have it absolutely backward: trust the dictionaries (written by lexicographers) and distrust google results (written by idiots of all degrees).
COCA, a reputable corpus, offers 48 instances of 'subarea' but only 4 instances of 'sub-area'.