Sorry if I'm being terribly stupid here, but I think this might be the combination of the selected source device being "alsa_output.pci-0000_00_1f.3.analog-stereo.monitor" (index: 0, denoted with an asterisk) and the way Zoom plays with microphones.
On a recently rebuilt Debian 10 laptop, I have source index 1 alsa_input.pci-0000_00_1b.0.analog-stereo as the default source and source index 0 alsa_output.pci-0000_00_1b.0.analog-stereo.monitor as a monitor/virtual device. I've changed the monitor/virtual device to be the default sink as per nicknack's setup with pacmd set-default-source 0 and I can reproduce the issue. If I revert back, it is fine again: pacmd set-default-source 1
I'm an absolutely novice when it comes to doing anything with PulseAudio, but it seems odd to me that the virtual/monitor device is the default source (input)? I could 100% missing the point.