System: Kubuntu 20.04 beta 64bits
There is an old report, but this still happens:
https://bugs.freedesktop.org/show_bug.cgi?id=93946

The problem sometimes happens when monitor enters power save mode or when I suspend/resume the system. The device is set to headphones by default when the problem occurs.

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My audio devices output:
$ pactl list sinks
Sink #0
        State: SUSPENDED
        Name: alsa_output.pci-0000_00_1b.0.hdmi-stereo
        Description: Built-in Audio Digital Stereo (HDMI)
        Driver: module-alsa-card.c
        Sample Specification: s16le 2ch 44100Hz
        Channel Map: front-left,front-right
        Owner Module: 6
        Mute: no
        Volume: front-left: 36700 /  56% / -15,11 dB,   front-right: 36700 /  56% / -15,11 dB
                balance 0,00
        Base Volume: 65536 / 100% / 0,00 dB
        Monitor Source: alsa_output.pci-0000_00_1b.0.hdmi-stereo.monitor
        Latency: 0 usec, configured 0 usec
        Flags: HARDWARE DECIBEL_VOLUME LATENCY SET_FORMATS 
        Properties:
                alsa.resolution_bits = "16"
                device.api = "alsa"
                device.class = "sound"
                alsa.class = "generic"
                alsa.subclass = "generic-mix"
                alsa.name = "HDMI 0"
                alsa.id = "HDMI 0"
                alsa.subdevice = "0"
                alsa.subdevice_name = "subdevice #0"
                alsa.device = "3"
                alsa.card = "0"
                alsa.card_name = "HDA Intel PCH"
                alsa.long_card_name = "HDA Intel PCH at 0xf7910000 irq 36"
                alsa.driver_name = "snd_hda_intel"
                device.bus_path = "pci-0000:00:1b.0"
                sysfs.path = "/devices/pci0000:00/0000:00:1b.0/sound/card0"
                device.bus = "pci"
                device.vendor.id = "8086"
                device.vendor.name = "Intel Corporation"
                device.product.id = "1e20"
                device.product.name = "7 Series/C216 Chipset Family High Definition Audio Controller (P8Z77-V LX Motherboard)"
                device.form_factor = "internal"
                device.string = "hdmi:0"
                device.buffering.buffer_size = "65536"
                device.buffering.fragment_size = "32768"
                device.access_mode = "mmap+timer"
                device.profile.name = "hdmi-stereo"
                device.profile.description = "Digital Stereo (HDMI)"
                device.description = "Built-in Audio Digital Stereo (HDMI)"
                module-udev-detect.discovered = "1"
                device.icon_name = "audio-card-pci"
        Ports:
                hdmi-output-0: HDMI / DisplayPort (priority: 5900, available)
        Active Port: hdmi-output-0
        Formats:
                pcm
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Mentioned workaround in old previous report still works:
https://askubuntu.com/questions/776883/pulseaudio-drops-hdmi-output-at-screen-power-save