How do I get an MCE IR remote to work with mpv?

I have an old Dell MCE remote which I would like to repurpose for remote controlling mpv.

I am running Kubuntu 20.04, with mpv version 0.32 and ir-keytable 1.18.0-2build1. I have set up ir-keytable with rc6_mce.toml in /etc/rc_keymaps. When I test the remote with ir-keytable -t, every key of the remote is correctly reported, 45 keys in total.

However, when I run mpv --input-test --force-window --idle and press the keys again, barely a handful of keys registers:

 input: Key DEL is bound to:
 input: 1.  'script-binding osc/visibility' in <builtin>:1 (default)
 input: Key DEL is bound to:
 input: 1.  'script-binding osc/visibility' in <builtin>:1 (default)
 input: Key ENTER is bound to:
 input: 1.  'playlist-next' in <builtin>:1 (default)
 input: Key RIGHT is bound to:
 input: 1.  'seek  5' in <builtin>:1 (default)
 input: Key LEFT is bound to:
 input: 1.  'seek -5' in <builtin>:1 (default)
 input: Key DOWN is bound to:
 input: 1.  'seek -60' in <builtin>:1 (default)
 input: Key UP is bound to:
 input: 1.  'seek  60' in <builtin>:1 (default)
 input: Key 0x21ff56 is bound to:
 input: (nothing)
 input: Key 0x21ff56 is bound to:
 input: (nothing)
 input: Key 0x21ff32 is bound to:
 input: (nothing)
 input: Key PLAY is bound to:
 input: 1.  'cycle pause' in <builtin>:1 (default)
 input: Key FORWARD is bound to:
 input: 1.  'seek 60' in <builtin>:1 (default)
 input: Key REWIND is bound to:
 input: 1.  'seek -60' in <builtin>:1 (default)
 input: Key CANCEL is bound to:
 input: (nothing)
 input: Key 0x21ff1c is bound to:
 input: (nothing)
 input: Key PAUSE is bound to:
 input: 1.  'cycle pause' in <builtin>:1 (default)

My input.conf shouldn't contain anything which interferes, it is not referenced in the input test either. For the sake of completeness:

input.conf

Shift+RIGHT frame-step
Shift+LEFT frame-back-step
R add sub-scale +0.1
Y add sub-scale -0.1
H seek -65
h seek 65
v cycle deband
a cycle audio
s cycle sub
i cycle interpolation
t script-message-to seek_to toggle-seeker
+ add audio-delay 0.010
- add audio-delay -0.010
F1 add sub-delay -0.1
F2 add sub-delay +0.1
F4 cycle-values video-aspect-override "16:9" "4:3" "2.35:1" "-1"

Furthermore, some keys like SLEEP, VOLUME_UP, VOLUME_DOWN and MUTE pass right through to the OS and do their thing, which can be seen by the KDE popup. (SLEEP is just logging me out, though).

To sum it up: 4 keys on the remote act on the system even when trying to input test, 4 keys just show a keycode (different to ir-keytable) even though ir-keytable recognized them perfectly, and just 11 keys are registered in mpv's input test. One key (DEL) seems to be assigned to two keys on the remote.

Also, when I try to add unassigned keys from the remote to input.conf, running mpv gives errors:

[input] Unknown key 'KEY_NEXT' at /etc/mpv/input.conf:17
[input] Unknown key 'KEY_PREVIOUS' at /etc/mpv/input.conf:18

There seems to be something missing to let mpv recognize the IR remote which is correctly set up in ir-keytable.

What do I need to do to have proper assignments of keys to mpv actions?