Do you have any power saving options on/off from your gpu ?
You can always try a different distro with a higher kernel version. Manjaro (arch based) uses the latest kernel.
Blank Screen after 15 mins enabled
Automatic suspend on battery enabled
Dim Screen when inactive enabled
These were the default settings I believe, I've never changed them.
Machine has been plugged in the whole time so none of the battery options should apply.
What gpu driver do you use ?
What command do I need to run to list that?
I previously ran lshw -c video earlier in this thread for detailed graphics listing.
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$ dmesg | grep -i amdgpu
What is the output from that ?
Should be noted I'm running kernel 5.8 while running this command and sleep is currently working (multiple sleep wake cycles)
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I found alot of users on Arch linux and on freebsb reporting the exact same issue as you have. I am 99% sure its you gpu.
On arch they fixed it by changing the kernel boot parameters
amd_iommu=on iommu=pt pci=noats
If the above did not work try booting with iommu=soft as kernel boot parameters
Here is a how to -> grub2 - How do I add a kernel boot parameter? - Ask Ubuntu
Sorry have not even been able to test it. Been working overtime 
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If you do have the time to test, let us know. 
I'm afraid to report that (from a clean install using the latest kernel) the kernel boot parameters made no difference with waking from sleep. The machine always gets stuck in sleep mode with both iommu=pt and iommu=soft. I'm going to change the machine back to using the older kernal again and see if that makes any difference.
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After additional testing on 5.11.0-40 I can confirm that the parameters have no effect on that kernel version either.
The only viable means of waking from sleep is to revert the kernel back to version 5.8.0-63-generic and set grub to automatically boot to this version of the kernel (I used grub customizer this time to set the predefined default entry to 5.8).
Waking from sleep seems to work *most of the time using this kernal but I also found that to improve reliability I had to change the power saving settings to turn off the screen blanker and turn off automatic suspend.
Aravisian's theory that a regression occurred between 5.11-0-40 and 5.8.0-63 seems to make the most sense at the moment as I can consistently get waking to work again by reverting to 5.8.0-63
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