I have a question regarding the pronunciation of a maksuur letter when it is attached to a saakin letter.

as-salaamu alaikum
I have a question regarding the pronounciation of a maksuur letter when it is attached to a saakinah.
Some qurraa tell me that there is no change in the sound that a maksuur letter should sound regardless of the condition/placement of it.
On the other hand other some other qurraa say that there is a difference.
example, when saying 'Alayhim . The side that says a maksuur is always the same, pronouncing it as 'Alay-hem (not stretched but e represents it being pronounced as a maksuur is normally pronounced when by itself like maaliki)
The other pronouncing a maksuur connected to a saakinah like alay-him (like we would say him in english)
so i am wondering what is correct, and if it is to say alay-hem,  then is it always like that? because i notice those qurraa who are most particular about the maksuur letters, never have that quality on - Ssiraata-al-latheena   (i always here it as Ssi not Sse . Also for banee israaeel, rather than esraaeel. or min khawf as apposed to men khawf.

Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The kasrah should sound the same no matter whether a saakin letter follows it or a voweled letter. The sound of the kasrah is the same sound as the letter , but only one vowel count.  A mistake that many beginning or non-trained tajweed students have is making an incomplete kasrah on a letter before a sukoon, thus making an incorrect sound like an "i" instead of the proper kasrah, which should sound like a  shortened

There is an agreed upon matter among the scholars of tajweed.

Wa iyyaaakum