Hi Arnold de Bruin,
Thanks for sharing updates with us. We feel sorry to hear that you meet such an issue. However, we appreciate your understanding.
First please accept my apologize for late reply.
I check with others and really this is abnormal behavior(delete expanded shortcut folder in OneDrive sync file explorer also deleted all files in SharePoint online document library)
Since, I also reproduce this issue from my side as well, the possible reason of this issue is related to the back-end, we suggest your contact IT admin of your organization and raise a ticket in Office 365 admin center-> Support-> New Service Request. Please don’t mind that we couldn’t provide further assistance because need to check from back-end side and we appreciate your efforts in this case. The support team will involve more resources to check from the back end and provide further help.
I appreciate your patience and understanding.
Best Regards
Waqas Muhammad