[[iuxta]]
Fragment of a discussion from User talk:Rua
FWIW, etymologically it looks like ū should be expected but French joste indicates that at least in Proto-Romance it was (probably) short (not necessarily in Classical Latin, cf. French annoncer < ad + nūntiāre with long ū). Also, some modern dictionaries appear to leave out long marks before two consonants so the short u in Langenscheidt might not be probative.