Pronunciation of ss as s and lack of sh?

Question

I've been hearing some British and Irish actors and presenters pronounce ss like s instead of sh, so tissue sounds like tisyu rather than tishu for example. I also heard someone pronounce appreSEEate instead of appreSHEEate. Now, I couldn't find much online about this apart from someone saying it's received English, but not much else. Anyone has further explanation or recommended reading on this. Would assured follow the same rule since it sounds unnatural to me, or passion?

Answer 1

This phenomenon is explained by a process that encompasses several principle and that is called "assimilation".

"Assimilation" is defined as follows in The Longman Pronunciation Dictionary (3rd edition, 2008).

([]: user LPH's comments)

(P. 159) Coarticulation
1 Speech sounds tend to be influenced by the speech sounds that surround them. Coarticulation is the retention of a phonetic feature that was present in a preceding sound, or the anticipation of a feature that will be needed for a following sound. Most ALLOPHONIC variation — though not all — is coarticulatory.
[…]
5 For cases where coarticulation is variable , and may result in what sounds like a different phoneme [one speaks of ASSIMILATION (p. 51)].

Assimilation (p. 51, 52)

(p. 51)
1 Assimilation is a type of COARTICULATION. It is the alteration of a speech sound to make it more similar to its neighbours. In English it mainly affects PLACE OF ARTICULATION.

(p. 52)
5 Yod coalescence (or 'coalescent' assimilation) is the process which changes t or d plus j ([yod]) into tʃ or dʃ respectively. Across word boundaries it may affect phrases involving you or your.
let you out              ˌlet ju ˈaʊt → ˌletʃ u ˈaʊt [no change in AmE]
would you try      ˌwʊd ju ˈtraɪ → ˌwʊdʒ u ˈtraɪ [no change in AmE]
get your bags      ˌget jɔː ˈbægz → ˌgetʃ ɔː ˈbægz || ˌget j^ər ˈbægz → ˌgetʃ ^ər ˈbægz [AmE]
6 Within a word, the status of yod coalescence depends on whether the following vowel is SRONG or WEAK.

• Where the vowel is strong, i.e. uː or əʊ, yod coalescence can frequently be heard in BrE, although not in careful RP. (In AmE there is usually no yod, so the possibility does not arise.)
  tune    tjuːn → tʃuːn
  endure    ɪn ˈdjʊə → ɪn ˈdʒʊə
• Where the vowel is weak, i.e. u or ə, assimilation is often variable in BrE, but obligatory in AmE.
  factual    ˈfækt ju‿əl → ˈfæk tʃu‿əl [isolated or careful pronunciation: ˈfækt ju əl → ˈfæk tʃu əl; usual: ˈfækt ju^əl → ˈfæk tʃu^əl]
  educate    ˈed ju keɪt → ˈedʒ u keɪt

It becomes now possible to understand # 7 in "Assimilation", this 7th part being that which explains the phenomenon referred to in the OP.

Assimilation, p. 52
7 Historically, a process of yod coalescence is the origin of the tʃ used by all speakers in words such as nature, and otf the dʒ in words such as soldier. Similarly, yod coalescence involving fricatives (sj → ʃ, zj → ʒ) explains the ʃ in words such as pressure, delicious, patient, Russian, and the ʒ in words such as measure.
For example, delicious came to English from Latin via the French délicieux de li sjø, but in English the j coalesced into ʃ several centuries ago.

Answer 2

In English pronunciation is notoriously inconsistent. Take, for example, '...ough': we have 'cough' (as in 'off'), 'enough' (as in 'cuff'), 'through' (as in 'true'), 'though' (as in 'throw'), 'thorough' (I cannot think of a word by the 'ough' sounds 'er'; 'thought' as in 'fort'; and probably others I have missed. Yours has to do with the shape of the mouth and palate.

There is an explanation in this case, which has to do with the origin of the word from French and the fact that in English, the letter 'y' is somewhere between a vowel and a consonant. To say 'tissue' in French the shape of the mouth stays the same except for a small movement of the tip of the tongue downwards, and with the lips slightly more pursed. So the movement is easy (if you are used to it. The French 'i' in 'tiss' sounds more like English 'teak' than 'tiss', which makes the mouth and tongue move further (uncomfortably) to make the 'you' sound. So many English (probably a majority) take a sort of short cut by pronouncing the 'ss' as 'sh'.

Pronunciation is a sequence of shapings of mouth, toungue and teeth and follows (in every language and dialect) its own law: follow the line of least resistance as you pass from one syllable to the next. So as the French word 'tissue', brought to us by the Norman aristocracy in 9th century, found its way in the Anglo-saxon and then English language, its pronunciation morphed. Just, by the way as the spelling of 'pronunciation' involves the dropping of the 'o' from 'pronounce', similarly following the line of least resistance. Today, 'tissue' as in 'kiss you' sounds a little old fashioned.