As guifa says in a comment, the sound change of /ks/ to /x/ was blocked when the letter *x* was followed by a consonant. In these circumstances /ks/ was generally reduced to /s/, with the /ks/ pronunciation re-emerging as a form of hypercorrection: >#-*X*- >The evidence indicates that this Latin letter represented [ks]. The development of the sound in Vulgar Latin and in the Romance languages varied from region to region, with more changes occurring in Spanish than in any other case. >In Castilian territory the [ks] > [js] > [sj] > [š]. This last phone leveled with [ž] from Latin J and G (e, i) in the fifteenth and sixteenth centuries, and the resulting [š] gradually became [χ] by 1650. In the nineteenth century the Spanish Royal Academy did away with the spelling *x*, except in Latinisms such as *examen, sexta,* and words such as *dixe* are now spelled with *j* (*dije*). In Mexico, especially, the *x* has been retained in words of Indian origin to represent a primitive [š] (Mexico, Xalapa, Oaxaca), now [χ], and in Yucatan one still hears the [š] (Uxmal) among the Maya- speaking inhabitants or among foreigners ... >... >**It is to be noted that the evolution of X to an ultimate [χ] did not take place when it was supported by a consonant** as in FRÁX(I)NU > *fresno*, SÉXTA > *siesta*, **and where X has been retained before a consonant, generally represents [s]** (*extraño* [estráño], *experiencia* [esperjénsja]).<sup>135</sup> >In the other Romance languages the -X- [ks] usually became a sibilant or retained its original Latin value. In Catalan and Portuguese the descendents are [s] (*x*) and [s] (*ss*) and the letter X has been extended to represent [š] of other origins than Latin X: ><sup>**135. In the latter type [s] (= *x* preceding consonant), hypercorrection often produces [ks] or [gs] through pedantic school teaching, i.e. *sexta* [séksta] and *extrano* [ekstráno]. This is not the popular pronunciation however.**</sup> >--- >Arabic [š] (P. *xadrez* C. *aixedres*), Latin -PS- (P. C. *caixa*). In Catalan the [č] is represented normally by *tx*.<sup>136</sup> >The -X- in Italy generally became [s] (ss), as it did in France, and the popular French orthographic *x* (*paix*, and the plurals *chateaux, cheveux*) simply represent a former final -s or -z written with a peculiar flourish so as to resemble somewhat an *x*.<sup>137</sup> >The prefix EX- in Italian and Romanian, by apheresis or loss of *e-* (along with prosthetic *e-*) was generally reduced to [s] or [z], depending on whether it was followed by a mute or voiced consonant: >- EXPÓNĚRE > Rm. *spune* [spúne], but It. *esporre* [espór̃e] >- ESCUTULĀRE > It. *scotolare* [sk-], Rm. *scutara* [sk-] >- EXVENTĀRE > It. *sventare* [zventáre], Rm. zvinta [zv-]; cf. Sard. *izventare*, C. Pr. *esventar* [-ez-], and Fr. *éventer* < OFr. *esventer*<sup>138</sup> >Before the front vowels *e* and *i*, however, Italian generally palatalized the [ks], spelled *sce*, *sci* [še, ši]. >Examples of standard treatments of -X-, showing the variety of treatments, are: >- EXEMPLU > It. *scempio* [s-], *esempio* [ez-], Sp. *ejemplo* [ex-], P- *exemplo* [ez-], C. *exemple* [egs-], Fr. *exemple* [egz-] >- EXPECTARE > It. *aspettare* [as-], Sard. *istettare* ... ><! > >[*An Introduction to Romance Linguistics*](https://books.google.co.uk/books?id=1WZfAAAAMAAJ&dq=%22x%20ks%22), Delos Lincoln Canfield, John Cary Davis (p.92-93)