The root of "human" إنسان (insaan) is ء ن س (ʾ-n-s)
The root of "forget" إنس (insa) is ن س ي (n-s-y)
Not the same root.
The word templates and roots are different, but the combination root1+template1 happens to look identical to root2+template2. (each arabic word is a 3-letter root + assigned into a large n-letter word template)
insa is the second-person masculine singular active imperative of نَسِيَ (nasiya), equivalent to the command "forget!" in english, addressed to male.
I think I found the root of this rumor [3] (some religious mumbo jumbo):
"In Egyptian Arabic, the word 'insan' means 'human'. If we remove the 'n', the word becomes 'insa', which means 'to forget'. So you see, the word 'forget' is taken from the word 'human'. And since it was God who created our minds and hearts, He knew from the very beginning that we would quickly forget our history"
refs:
[1] human - https://en.wiktionary.org/wiki/%D8%A5%D9%86%D8%B3%D8%A7%D9%8...
[2] forget - https://en.wiktionary.org/wiki/%D9%86%D8%B3%D9%8A
[3] https://www.goodreads.com/quotes/7942501-in-egyptian-arabic-...
Note: I corrected my comment - previously I spelled the second word incorrectly.