I've tried this several times, all using M365 personal (white cloud) and the folders in question are shared with a combination of paid users, users under my family account and free users. Across all this there is a mix combination of who can see the file in Explorer and who is now restricted to online only.
One user who is under my paid family account had this issue from the beginning almost 9 months ago, every possible fix and attempt has been tried with them to no fix. In the past month another paid user who several folders shared with had this happen. Then over the weekend (which seems to be where many have suddenly had issues) the second user under my paid family account had all their folders turned to .url instead. Again tried new folders and fixes and no success.
To note, all the folders I've shared with several different accounts are all private, there is no business M365 in use. They have all lost access at different times over the past 9 months. Also to note every folder is always and has always had full read-write access as it's all team based work where everyone needs access.
The inability to share locally folders between M365 Family and M365 Business is ridiculous but I can understand, but are we saying that if you have the free version of OneDrive you are restricted to online only, no File Explorer ability?
Also on all this and the mention that the issue is fixed, Microsoft still have this officially listed as a bug, so is there communication from Microsoft detailing somewhere more specifically that this whole lengthy process is required.