Web parts error message for guest users

Update:
After my case was escalated to the SharePoint tech team in Microsoft, they discovered that there was a customised script added at the tenant level, stored in a sharepoint site called CDN. This site was created to store the script files after activating Office 365 CDN for the whole tenant and that way implement the Tenant Wide Deployment option for SharePoint Framework (SPFx) Extensions, which allows the use of the scripts.

For some reason the above is messing with the way guest users see web-parts and generating the bug.
The possible solutions Microsoft presented are two:
1. Remove the customised script (after assessing the impact that this removal may have in other sites in the tenant)
2. Grant read-only access to the guest users to the CDN sharepoint site where the scripts are stored

In our case, because we still need the scripts as they are part of a previous implementation that is currently being used, we decided to go with option 2.

I've tested adding a guest user to a group with read-only access to the CDN site where the scripts are, and now the guest user doesn't experience the bugs anymore.

Hope the above helps, and if you have any questions just flick me a reply.

Regards,
Mario