iMac won't sleep automatically since macOS Catalina
Since updating to macOS Catalina, my iMac will not sleep automatically correctly.
When Power Nap is enabled, it will go to sleep after the period set in System Preferences/Energy Saver. It then wakes almost immediately for roughly a minute, then sleeps. It then fails to sleep after the first time it wakes to do its backup/sync tasks.
When Power Nap is disabled, it simply fails to sleep automatically at all.
It will sleep properly and seemingly indefinitely if I put it to sleep manually via keyboard shortcuts or the apple menu.
I have tried everything I can think of or have found suggested in online searches or in Apple's help pages, even to the extent of a clean reinstall of macOS Catalina 10.15.3. With no data restored or non native software installed, no external peripherals connected other than Ethernet, it behaves exactly the same way, which suggests this is not being caused by software I have installed, or external hardware (USB 3 HD and Audio/MIDI interface). It just will not sleep automatically.
This machine and exact configuration worked just fine under macOS Mojave.
For reference,
iMac Retina 5K, 27-inch, Late 2015)
Processor 3.2 GHz Quad Core Intel Core i5
Memory 32GB 1867 MHz DDR3
Startup Disk Macintosh HD
Graphics AMD Radeon R9 M390 2GB
I have 764GB available of the original 1.02TB Fusion drive available, the hardware is all as supplied and all software is currently up to date.
Anyone have any idea what's going on? It's not the end of the world but I prefer not to power down manually and would rather it sleeps automatically when idle. I have found a couple of threads from others online with similar issues but nothing anyone has suggested so far has made any difference.
Many thanks!
iMac 27", macOS 10.15