5600X idle temps 'issue'

[Captain303]

Hi everyone - hoping I can get some insight as to what's going on here...

 

Recently bought a Ryzen 5 5600X with a Noctua NH-L9x65 SE-AM4 cooler on an MSI X570-A Pro and set the power profile to 'balanced' as per recommendations.
It was happily sitting at idle temps of 32 degC until about a week ago.
Now, when the machine is left for a little while, idle temps sit at around 47-52 degC but here's the bizarre bit - if I bring up task manager* and close it straight away the temps start dropping down to 32 again. Within a few minutes they've crept back to 47...

 

* "System Interrupts" and/or "System" show top CPU% for a split second, with Power Usage highlighted in red showing "very high" and disappears immediately.
 

Under load games hit around 55-66 degC (Cyberpunk 2077, Project Cars 3, Read Dead Redemption 2, Ghost Recon Wildlands). Prime95 torture test hit 80. When I exit from any load-based application temps drop to 32 after a few seconds. Again, give it a few minutes and they rise back up to 47...

I'm sure there was a Windows update installed that coincides with the start of this behaviour, but the power profile advanced settings seem unchanged.

 

Can anyone shed any light as to what's happened and is there something I can tweak to get back the cooler idle temp?