OK here is my Hypothesis.
When installing the Sim, the initial install from MS-store asks you where you would like to install. As I have a dedicated NVMe2 drive (C) for Windows I wanted to install on a dedicated 1Tb NVMe2 drive (I) for MSFS. so selected that drive.
Checking my installation folders last night I realise that this installs the core package on the selected drive with linked folders on the C drive.
As previously stated my community folder is separate on the (I) and I wondered if all this linking is causing issues?
Checking the Zendesk info as per your suggestion they strongly advise installing the core 1.3gb program to the (C) drive and THEN setting up a separate folder called MSFS or something on the other (I) and selecting that when prompted to download the content from the actual SIM.
This will eliminate one set of links for sure and hopefully impact my issue.
And so, as you may have suspected by now I have done a complete un-install and am re-installing a clean version on the (C) drive with content only on the (I) drive.
This will take all day but hey well worth the effort.
If this work then it highlights a floor in their installation process when it gives you the option to install on another drive at the Package level as this ( as the Zendesk article says) may cause issues down the road!!
Will get back to you when up and running and theory tested.