Bold Italic and Bold Italic not automatically discoverd for IBM Plex Mono

[CharString]

Describe the bug
font_family IBM Plex Mono Text or (without Text, which uses IBMPlexMono-Regular) doesn't detect the Bold nor Italic variants:

chars@trailblazer:~$ kitty --debug-font-fallback true
[247 11:21:24.965169] Preloaded font faces:
[247 11:21:24.965238] normal face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-Text.otf:0
[247 11:21:24.965275] bold face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-Text.otf:0
[247 11:21:24.965305] italic face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-Text.otf:0
[247 11:21:24.965329] bi face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-Text.otf:0

The variants are mentioned in list-fonts, so setting them manually with bold_font, italic_font and bold_italic_font is a workaround:

chars@trailblazer:~$ kitty --debug-font-fallback true
[247 11:22:03.156231] Preloaded font faces:
[247 11:22:03.156310] normal face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-Text.otf:0
[247 11:22:03.156352] bold face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-Bold.otf:0
[247 11:22:03.156394] italic face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-TextItalic.otf:0
[247 11:22:03.156432] bi face: /home/chars/.local/share/fonts/IBM-Plex/IBM-Plex-Mono/IBMPlexMono-BoldItalic.otf:0

For font_family JetBrains Mono the variants are correctly detected by kitty. So I tried replacing the ttf from Debian 10.5 with the latest otf release from the IBM Plex github.

gnome-terminal does detect the variants automatically for IBM Plex. .

To Reproduce
Steps to reproduce the behavior:

1. Install IBM Plex Mono fonts
2. confirm kitty sees all variants as valid monospace fonts with kitty list-fonts
3. add font_family IBM Plex Mono to rc-file
4. kitty --debug-font-fallback

Expected behavior
Variants to be correctly set when just setting font_family like kitty does for other fonts.

Environment details

Details

kitty 0.18.3 (601d37aa3d) created by Kovid Goyal
Linux trailblazer 4.19.0-10-amd64 #1 SMP Debian 4.19.132-1 (2020-07-24) x86_64
Debian GNU/Linux 10 \n \l
Loaded config files: /home/chars/.config/kitty/kitty.conf
Running under: X11

Config options different from defaults:
background           Color(red=0, green=43, blue=54)
bell_on_tab          False
color0               Color(red=7, green=54, blue=66)
color1               Color(red=220, green=50, blue=47)
color10              Color(red=88, green=110, blue=117)
color11              Color(red=101, green=123, blue=131)
color12              Color(red=131, green=148, blue=150)
color13              Color(red=108, green=113, blue=196)
color14              Color(red=147, green=161, blue=161)
color15              Color(red=253, green=246, blue=227)
color2               Color(red=133, green=153, blue=0)
color3               Color(red=181, green=137, blue=0)
color4               Color(red=38, green=139, blue=210)
color5               Color(red=211, green=54, blue=130)
color6               Color(red=42, green=161, blue=152)
color7               Color(red=238, green=232, blue=213)
color8               Color(red=0, green=43, blue=54)
color9               Color(red=203, green=75, blue=22)
cursor               Color(red=131, green=148, blue=150)
enable_audio_bell    False
font_family          IBM Plex Mono Text
font_size            10.0
foreground           Color(red=131, green=148, blue=150)
inactive_text_alpha  0.8
placement_strategy   top-left
selection_background Color(red=88, green=110, blue=117)
selection_foreground Color(red=0, green=43, blue=54)
shell                xonsh --shell-type=rl
visual_bell_duration 0.1
window_padding_width FloatEdges(left=1.0, top=1.0, right=1.0, bottom=1.0)
Added shortcuts:
	control+l KeyAction(func='combine', args=(KeyAction(func='clear_terminal', args=['scroll', True]), KeyAction(func='send_text', args=['normal,application', b'\x0c'])))
	super+comma KeyAction(func='edit_config_file', args=())
Changed shortcuts:
	shift+control+0 KeyAction(func='change_font_size', args=(True, None, 1.0))