Only keypress is recognized, no key release

Question

I've been trying for quite some time to map my audio controls to the corresponding XF86 keys. However,three keys: XF86AudioLowerVolume, XF86AudioRaiseVolume & XF86AudioMute refuse to work properly. After each login, I can press each of these keys exactly once. Afterwards, their input is repeated until I press another key or send another key signal. xev also shows a repeating keypress from these keys, and showkey shows only a keypress event, but no release. To clarify, this is what the output looks like:

keycode 113 press
keycode 114 press
keycode 115 press
 keycode  57 press
keycode  57 release

You can see that the spacebar (keycode 57) shows a release as expected, but keycodes 113-115 don't. Now I have looked all around on the internet, but I haven't seen another post in which only the key press is registered, and not the key release. I recently upgraded to ubuntu 19.10 and reinstalled xorg but to no avail. Any suggestions?

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EDIT:

For all of the three keys, I only get an output in xev the first time I press it; the key is just repeated over and over again, like this:

KeyPress event, serial 34, synthetic NO, window 0x1e00001,
    root 0x168, subw 0x0, time 35686, (441,184), root:(1725,204),
    state 0x0, keycode 121 (keysym 0x1008ff12, XF86AudioMute), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 34, synthetic NO, window 0x1e00001,
    root 0x168, subw 0x0, time 36347, (441,184), root:(1725,204),
    state 0x0, keycode 121 (keysym 0x1008ff12, XF86AudioMute), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False
        ........

What surprises me is the line XLookupString gives 0 bytes:, that can't be right? Normal keys a single byte here, which corresponds to the output character. Other XF86 keys give me events like this:

KeymapNotify event, serial 157, synthetic NO, window 0x0,
    keys:  2   0   0   0   0   0   0   0   0   0   0   0   0   0   0   14  
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

Also, the keycodes that show up in xev to correspond to the correct bindings when I run xmodmap -pke. I am inclined to believe that the problem isn't with my X server, because restarting the X server with sudo systemctl restart display-manager does nothing for me, neither does relogging.

I've also tried restarting the keyboard with

xinput disable 'AT Translated Set 2 keyboard'; 
xinput enable 'AT Translated Set 2 keyboard'

but to no avail.

I've tried restarting the atkbd module (the keyboard controller) with commands such as modprobe -r atkbd only to receive messages like modprobe: FATAL: Module atkbd is builtin.

So far, the only way to press (and never release) those keys is to restart.

Perhaps it is important to tell that I run a dual booted system with GRUB2. In Windows, the keys work just fine. I guess they also do in the bios, although that's a little hard to test for volume keys.

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EDIT 2:

I am using a Medion Erazer P6661 laptop with built-in keyboard. By listing my keyboard with xinput list-props 12, I get the following information:

Device 'AT Translated Set 2 keyboard':
    Device Enabled (167):   1
    Coordinate Transformation Matrix (169): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000
    libinput Send Events Modes Available (289): 1, 0
    libinput Send Events Mode Enabled (290):    0, 0
    libinput Send Events Mode Enabled Default (291):    0, 0
    Device Node (292):  "/dev/input/event4"
    Device Product ID (293):    1, 1