Hello again
@vulgo and
@ekohn00 and
@markmac and everyone else with this problem.
I dived into the Framework suggested by
@vulgo and the Info.plist to check it out.
Basically it contains a list of Board IDs and a "key" or toggle for:
<key>forceOfflineRenderer</key>
<true/>
<key>jpeg</key>
<true/>
... under each Board ID (well most)
The iMac14,2 System Definition Board ID I was using *is* listed in the plist with the above key.
At first I wasn't sure how to read this so then checked the MacPro6,1 I had changed to as I mention before.
The MacPro6,1 Board ID *is not* listed.
So I copied the Info.plist and removed the section for the iMac14,2 Board ID, saved the plist and copied it back into the Framework (you need to do this carefully as you'll be required to verify each deletion and name change you do).
Anyway, without the iMac14,2 board ID in there I now have everything back to as it was before and Preview and Quicklook now work perfectly with JPEGs.
Thank you for the pointer
@vulgo
Quite why Apple have done this is confusing. Why would they disable hardware rendering of JPEGs, or any other files for that matter, on so many quite recent Board IDs?