Same bug apparently and i have ubuntu focal kernel. In my case the next command this fix partially my problem, because my audio is set to null in sound, but i can listen it:
pacmd set-card-profile 0 output:hdmi-stereo

Note i have this problem for a long time, but with ubuntu focal is worse than ever. I can not find a good permanent solution.

Only in pavucontrol i can see my HDMI Audio and this is what i can see when i can lisent something:

Card: RS880 HDMI Audio [Radeon HD 4200 Series]
Profile: Digital Stereo (HDMI) Output (unplugged) (unavailable)
Internal Audio: off

pacmd list-cards
2 card(s) available.
    index: 0
	name: <alsa_card.pci-0000_01_05.1>
	driver: <module-alsa-card.c>
	owner module: 7
	properties:
		alsa.card = "1"
		alsa.card_name = "HDA ATI HDMI"
		alsa.long_card_name = "HDA ATI HDMI at 0xfdafc000 irq 19"
		alsa.driver_name = "snd_hda_intel"
		device.bus_path = "pci-0000:01:05.1"
		sysfs.path = "/devices/pci0000:00/0000:00:01.0/0000:01:05.1/sound/card1"
		device.bus = "pci"
		device.vendor.id = "1002"
		device.vendor.name = "Advanced Micro Devices, Inc. [AMD/ATI]"
		device.product.id = "970f"
		device.product.name = "RS880 HDMI Audio [Radeon HD 4200 Series]"
		device.string = "1"
		device.description = "RS880 HDMI Audio [Radeon HD 4200 Series]"
		module-udev-detect.discovered = "1"
		device.icon_name = "audio-card-pci"
	profiles:
		output:hdmi-stereo: Digital Stereo (HDMI) Salida (priority 5900, available: no)
		output:hdmi-surround: Digital Surround 5.1 (HDMI) Salida (priority 800, available: no)
		output:hdmi-surround71: Digital Surround 7.1 (HDMI) Salida (priority 800, available: no)
		off: Apagado (priority 0, available: unknown)
	active profile: <output:hdmi-stereo>
	sinks:
		alsa_output.pci-0000_01_05.1.hdmi-stereo/#1: RS880 HDMI Audio [Radeon HD 4200 Series] Digital Stereo (HDMI)
	sources:
		alsa_output.pci-0000_01_05.1.hdmi-stereo.monitor/#1: Monitor of RS880 HDMI Audio [Radeon HD 4200 Series] Digital Stereo (HDMI)
	ports:
		hdmi-output-0: HDMI / DisplayPort (priority 5900, latency offset 0 usec, available: no)
			properties:
				device.icon_name = "video-display"
    index: 1
	name: <alsa_card.pci-0000_00_14.2>
	driver: <module-alsa-card.c>
	owner module: 8
	properties:
		alsa.card = "0"
		alsa.card_name = "HDA ATI SB"
		alsa.long_card_name = "HDA ATI SB at 0xfe024000 irq 16"
		alsa.driver_name = "snd_hda_intel"
		device.bus_path = "pci-0000:00:14.2"
		sysfs.path = "/devices/pci0000:00/0000:00:14.2/sound/card0"
		device.bus = "pci"
		device.vendor.id = "1002"
		device.vendor.name = "Advanced Micro Devices, Inc. [AMD/ATI]"
		device.product.id = "4383"
		device.product.name = "SBx00 Azalia (Intel HDA)"
		device.form_factor = "internal"
		device.string = "0"
		device.description = "Audio Interno"
		module-udev-detect.discovered = "1"
		device.icon_name = "audio-card-pci"
	profiles:
		input:analog-stereo: Estéreo analógico Entrada (priority 65, available: no)
		output:analog-stereo: Estéreo analógico Salida (priority 6500, available: no)
		output:analog-stereo+input:analog-stereo: Estéreo Analógico Dúplex (priority 6565, available: no)
		output:analog-surround-21: Análogico envolvente 2.1 Salida (priority 1300, available: no)
		output:analog-surround-21+input:analog-stereo: Análogico envolvente 2.1 Salida + Estéreo analógico Entrada (priority 1365, available: no)
		output:analog-surround-40: Analógico envolvente 4.0 Salida (priority 1200, available: no)
		output:analog-surround-40+input:analog-stereo: Analógico envolvente 4.0 Salida + Estéreo analógico Entrada (priority 1265, available: no)
		output:analog-surround-41: Analógico envolvente 4.1 Salida (priority 1300, available: no)
		output:analog-surround-41+input:analog-stereo: Analógico envolvente 4.1 Salida + Estéreo analógico Entrada (priority 1365, available: no)
		output:analog-surround-50: Analógico envolvente 5.0 Salida (priority 1200, available: no)
		output:analog-surround-50+input:analog-stereo: Analógico envolvente 5.0 Salida + Estéreo analógico Entrada (priority 1265, available: no)
		output:analog-surround-51: Analógico envolvente 5.1 Salida (priority 1300, available: no)
		output:analog-surround-51+input:analog-stereo: Analógico envolvente 5.1 Salida + Estéreo analógico Entrada (priority 1365, available: no)
		output:analog-surround-71: Analog Surround 7.1 Salida (priority 1200, available: no)
		output:analog-surround-71+input:analog-stereo: Analog Surround 7.1 Salida + Estéreo analógico Entrada (priority 1265, available: no)
		output:iec958-stereo: Estéreo Digital (IEC958) Salida (priority 5500, available: unknown)
		output:iec958-stereo+input:analog-stereo: Estéreo Digital (IEC958) Salida + Estéreo analógico Entrada (priority 5565, available: no)
		off: Apagado (priority 0, available: unknown)
	active profile: <off>
	ports:
		analog-input-front-mic: Micrófono frontal (priority 8500, latency offset 0 usec, available: no)
			properties:
				device.icon_name = "audio-input-microphone"
		analog-input-rear-mic: Micrófono trasero (priority 8200, latency offset 0 usec, available: no)
			properties:
				device.icon_name = "audio-input-microphone"
		analog-input-linein: En línea (priority 8100, latency offset 0 usec, available: no)
			properties:
				
		analog-output-lineout: Línea de salida (priority 9000, latency offset 0 usec, available: no)
			properties:
				
		analog-output-headphones: Auriculares analógicos (priority 9900, latency offset 0 usec, available: no)
			properties:
				device.icon_name = "audio-headphones"
		iec958-stereo-output: Salida digital (S/PDIF) (priority 0, latency offset 0 usec, available: unknown)
			properties: