As you have probably seen the very recent question Why are “south” and “southern” pronounced with different vowels?
The only (excellent) answer suggests that it is because of "Trisyllabic Laxing". (explained there in detail)
My question is: if the so-called trisyllabic laxing shortened the vowel in "southern", why doesn't it shorten the vowels in "northern" and "eastern"?
They also have the suffix "ern" and they would also be three syllables words in Old English (like "southern" was).
They have long vowels in their corresponding words "north" and "east" too. Is this irregular or there is an explanation for this?