Why was the lām of al-taʿrīf assimilated into the shīn but clearly pronounced before the jīm, even though both letters (jīm and shīn) share the same point of articulation, and the reason for assimilation is proximity in articulation while the reason for clear pronunciation is distance in articulation?

"Idgham" (merging) has three causes: the proximity of the two letters in their articulation point (makhraj), their homogeneity through agreement in characteristics (sifat), or their complete similarity. The letter "lam" is not merged into "jim," unlike "shin," because "jim" has a more distant articulation point and less agreement in characteristics with "lam." Indeed, "jim" is a strong letter characterized by loudness (jahr) and strength (shidda), which is contrary to "lam."