It seems like DictionaryLookup was not designed to work with Alternatives of StringExpression in earlier versions and returns only the result for the first alternative.
Instead it works with Alternatives applied to a single characterscharacter.
First line works with "e" | "t" but fourth line DictionaryLookup[(x_ ~~ x_ ~~ y_) | (x_ ~~ y_ ~~ y_)] returns result like if it was just DictionaryLookup[(x_ ~~ x_ ~~ y_)].
DictionaryLookup[x_ ~~ "e" | "t"]
DictionaryLookup[x_ ~~ x_ ~~ y_]
DictionaryLookup[x_ ~~ y_ ~~ y_]
DictionaryLookup[(x_ ~~ x_ ~~ y_) | (x_ ~~ y_ ~~ y_)]
{"at", "be", "he", "it", "Le", "me", "re", "we", "ye"}
{"aah", "BBC", "DDT", "eek", "eel", "LLB", "ooh", "ppm", "ssh", \
"WWW", "XXL"}
{"add", "all", "Ann", "ass", "baa", "bee", "boo", "brr", "CNN", \
"coo", "Dee", "ebb", "eff", "egg", "ell", "err", "fee", "gee", "goo", \
"ill", "inn", "lee", "Lee", "loo", "moo", "nee", "odd", "off", "Orr", \
"pee", "see", "shh", "tee", "too", "wee", "woo", "WWW", "zoo"}
{"aah", "BBC", "DDT", "eek", "eel", "LLB", "ooh", "ppm", "ssh", \
"WWW", "XXL"}
As a workaround you can use:
DictionaryLookup /@
List @@ ((x_ ~~ x_ ~~ y_) | (x_ ~~ y_ ~~ y_)) // Flatten
{"aah", "BBC", "DDT", "eek", "eel", "LLB", "ooh", "ppm", "ssh", \
"WWW", "XXL", "add", "all", "Ann", "ass", "baa", "bee", "boo", "brr", \
"CNN", "coo", "Dee", "ebb", "eff", "egg", "ell", "err", "fee", "gee", \
"goo", "ill", "inn", "lee", "Lee", "loo", "moo", "nee", "odd", "off", \
"Orr", "pee", "see", "shh", "tee", "too", "wee", "woo", "WWW", "zoo"}