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As Salamu Alaykum:
Wa alaikum assalaam wa rahmatu Allahi wa barakatuh. May Allah keep our intentions solely for Him and guide us all in all that we do.
Your observations are correct, some Muslims in different parts of the world mispronounce the letter and it sounds more like a "Zaa" than the correct sound. This is not acceptable, and those of knowledge need to explain to those that do not have the knowledge the correct way of pronouncing this letter (and any other mispronounced letters). This type of mistake falls into the category of , or clear and obvious mistakes. These are mistakes that are not acceptable and change the Qur'an. Indeed, these mistakes are serious and ones that must be avoided.
There is no qira'ah that pronounces the letter like a Zaa'. All of the qira'aat pronounce the letter the same way.
It is a common mistake because
is a letter unique to the Arabic language and
one that needs a bit of practice for the non-Arab to perfect, but also the
colloquial Arabic, and most likely the other languages use the sound you
referred to ( a Zaa with tafkheem) in their everyday speaking in
place of the .
Wa iyyakum wa-l-muslimeen.