I'm on Kubuntu 20.04; I found that my xmodmap wasn't mapping F13 through F24 (i.e., the window system is never supplying those keypresses). Once I xmodmapped the keys to actually be F13 through F24 (instead of e.g. XF86Activity5) they've generally worked.
(My copy of obs-studio on Kubuntu doesn't seem to recognize the XF86* Keycodes, which is what F13 through F24 were sending.)
Here's the contents of my $HOME/.Xmodmap file that gets F13 through F25 working for me on Kubuntu:
Code:
! XF86Mail
keycode 163 = F25 F25 F25
keycode 191 = F13 F13 F13
keycode 192 = F14 F14 F14
keycode 193 = F15 F15 F15
keycode 194 = F16 F16 F16
keycode 195 = F17 F17 F17
keycode 196 = F18 F18 F18
keycode 197 = F19 F19 F19
keycode 198 = F20 F20 F20
keycode 199 = F21 F21 F21
keycode 200 = F22 F22 F22
keycode 201 = F23 F23 F23
keycode 202 = F24 F24 F24
Note that you may need to log out and back in for the new settings to take effect, or run "xmodmap ~/.Xmodmap" to get the new settings loaded.
Pm