I have just tried the latest nightly build (LibreELEC-Generic.x86_64-10.0-nightly-20210409-d6594c8) and it is still the same.
I wanted to make trying nightly builds easier by putting the nightlies on a usb stick and booting from that so I don't need to keep updating and reverting the SSD. To do that I had to remove the htpc from its cabinet and disconnect the SSD to get it to go into the BIOS as it wasn't setup to boot from usb. While I had it out I removed the AMD RX 550 and tried the same test just using the intel 3770k iGPU and still had exactly the same issue.
Could the AMD card be a red herring and it's one of the other PC components on the motherboard causing the issue or the Hauppauge Quad HD card? That's basically all there is in the case, or is it pretty certain it will be related to a GPU in one form or another?
This has only been n issue since libreelec 10.0, I had tried 9.80 nightlies previously and they worked fine with my hardware.
Is there anything else I can do/test to give you more information on this?
Thanks
*EDIT*
I have uploaded a file with DMESG / LSPCI and LSUSB outputs. These are from LibreELEC-Generic.x86_64-10.0-nightly-20210409-d6594c8 booted from USB stick :