The scan of Apps installed reports 'calibre 64bit - 5.17.0' as not installed.
The list of Apps however only shows 'calibre 5.18(x86)' and therefore won't load the 64 bit version.
It is permissible to install both versions although I only use the 64 bit version now.
The option 'Install 32-bit Apps On New Installs' is NOT selected so in theory it should load the 64 bit version by default.
This means that you have (5.17 x86) installed with (5.18 x64) too !
So make sure to remove (5.17 x86), It looks like it didn't remove when you switched to (5.18 x64)
Yes, I had 2 copies installed, similarly with Thunderbird. Both were 32 bit programs but they both introduced a new 64bit experimental version alongside their 32 bit version and were interchangeable without affecting the data in case of problems with the 64bit version.
I only use the 64bit versions now but forgot to eventually uninstall the 32bit versions. As a consequence these 2 programs have not been updating when Patch My PC runs, I always did them manually.
I was able to simply uninstall the 32 bit Thunderbird software successfully and it now updates to the 64bit software.
Calibre however doesn't. I uninstall calibre 5.18 (X86) but Patch My PC simply reinstalls the 32bit software again.
As far as I can see the list of software definitions cannot be changed by myself in order to locate the 64bit version of calibre download
It seems that I have answered my own question about updating the list of Apps.
After uninstalling Calibre 5.18 (X86), you need to close Patch My PC and then restart the programme.
It the then finds and adds 'Calibre 5.18 (X64) to the list of Apps instead of Calibre 5.18 (X86)