Both negate, but they do not produce the same form or time meaning.
● وَإِذْ قُلْتُمْ يَا مُوْسَى لَنْ نَصْبِرَ عَلَى طَعَامٍ وَاحِدٍ — “We will not endure one kind of food”: لَنْ gives future negation and نَصْبِرُ ends in fatḥah. (Al-Baqarah 2:61)
● لَنْ يَنْفَعَكُمُ الْفِرَارُ — Flight will never benefit you: strong future negation; final fatḥah.
● وَإِذًا لَا تُمَتَّعُونَ إِلَّا قَلِيلًا — Then you will be allowed to enjoy only a little: لَا does not remove the inflectional nūn here. (Al-Ahzab 33:16)
Do not change a verb merely because you see any negative particle; first identify the particle itself.