Order of Correlative Conjunction + Preposition

From the Cambridge Dictionary of the English Language:

https://dictionary.cambridge.org/us/grammar/british-grammar/either-or

We use either… or… to connect items which are the same grammatical type, e.g. words, phrases, clauses:

We can either pre- or post-date the document. I don’t mind. (connecting prefixes)

It’s either black or grey. I can’t remember. (connecting words)

You can stay either with me or with Janet. (connecting phrases)

“I hope to see you either in France or Belgium” violates this rule, because “in France” is not parallel to “Belgium,” and in effect you’ve said “I hope to see you Belgium.”

You can say “I hope to see you either in France or on the boat,” because both “in” and “on” are prepositions.