So i tested this on windows as well as with pop!_os (my distro) DE and another DE, and only on cinnamon i'm observing this.

I'm on a german notebook and set up brazilian keyboard layout which, again, in the other contexts is working fine. But on Cinnamon, always one of the following keys will not work byitself, but will when shift is held down: minus/underline, closing square bracket/ closing curly bracket, backslash/vertical bar. As of this moment, it's the last of those which presents this problem. backslash won't appear, but when pressed with shift held down, will give a vertical bar |

I don't know actually if the problem was there from the installation moment or if it began when i started making attempts to remap the last of those keys to slash/question mark, which is also a necessity i need help with.

I tried using input-remapper, xev, but to no avail.

PC: HP Pavilion 15, pop!_OS 22.04 jammy, Cinnamon version 5.2.7

Would be nice to explain better what did you try and exactly what's happened? I would suggest to use xev to test your keyboard events.
You may try to switch to a console and check if that works.
You may check the logs probably you will find something useful.

i entered xev | grep key

the output for the key that is currently not working, which is closing square bracket, is this

keys: 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

the output for a key that is working, which is \ and | with shift is this

state 0x2010, keycode 94 (keysym 0x5c, backslash), same_screen YES,
XKeysymToKeycode returns keycode: 51

again, when i press the closing square bracket with shift, it produces } normally.

any idea from that?