We know that letter x is pronounced "cs" ("ks") i.e. as two consonants. But it is still one letter. When it stands between two vowels, is the first syllable considered open or close?
Closed. As X stands for CS, an X between two vowels means that there are two consonants between them and therefore the preceding syllable is always heavy/long.
Sydney Allen in Vox Latina mentions this indirectly. In addition to pointing out that X is CS, he mentions that this leads to a case of hidden quantity, and earlier mentions that hidden quantity is a term for the vowel length in closed syllables.