[Galaxy Note 4 Android Development (Exynos)] :
Okay. So here's what I've discovered so far...

Before on android 5.1, 5.0 and 4.4 samsung devices had fingerprints sensors but android did NOT support them. So Samsung made its own Fingerprint Service which required a backup password ONLY and my guess is that that system back on 5.1.1 was the same as the one on the note4 and that's why Note5 ported roms with 5.1.1 had no issues with fingerprint. Now, on 6.0, android fully supports fingerprint sensors and when it's set as the screen lock type, it can have a backup pin, pattern or password and not just the password. Now, samsung did implement this feature for the s7, s6 and note5 but didn't use it for the note4 on MM, so the Note4 still uses the same fingerprint service and method as kitkat and only supports a backup password. Now, porting the new fingerprint service I can say was impossible or very hard for the devs, obviously so they replaced it with the old one from the Note4 so that atleast fingerprint could work with 3rd party apps. The new S7 settings and fingerprint service apps use the new drivers which are only compatible with the new touch fingerprint sensors and that's why you can't set screen lock type to fingerprint. I managed to set it to Fingerprint by manually modifying Lockscreen database but the problem is that now, you can only unlock the screen using the backup pin or password. My guess is that either the old fingerprint service app stores the fingerprint data in some place other than S7's fingerprint storage. What I need now is to find out where Galaxy s7 and where the Note4 store their fingerprint data. It wasn't in the fingerprint service app, but if I find it, I might have a shot at fixing it. Another theory I have is that the problem is caused by the lockscreen itself which is inside SystemUI. Since I'm sure everyone remembers, Good Lock which is basically S7's SystemUI broke fingerprint unlock on stock MM on the Note4. I also tried replacing the lockscreen database with the one from stock MM with fingerprint set but that made systemui crash everytime I entered backup password.

Now, in Lockscreen.db file, there's a row called lockscreen_type and it's a random-looking number, I analyzed the number and it turns out when my lockscreen is set to PIN the number's hexadecimal is 30000, when set to pattern it's hex is 20000 and password is 40000. I only managed to get my hands on this DB once before it was encrypted. Right now, no matter how hard I try, it is encrypted and I can't open it so I had to use the old one and figure out how to change it. Anyways, I changed the screen lock type to password hoping that when it is set to the same password as my fingerprint's backup password it would work, but still, nothing. I do get the massage saying I should use my fingerprint to unlock, but I can only unlock with my backup password. At least I've made progress. Right now, I think it's 60 percent change that SystemUI has a problem with the old fingerprint service and 40 percent chance that I missed something. So pray, because I think if I try a little harder, I might actually be able to fix it.

EDIT: I'm now sure that the problem is not SystemUI. As I mentioned, Good Lock didn't work on Stock MM when fingerprint was on, well, I tried installing Good Lock now and it worked. That means that fingerprint's problem is definitely because of either Fingerprint Service which is old.

Some Screenshots: