Many thanks, Hans, for clarifying this very unfortunate issue.
I have two machines on my own account, with a M365 family subscription, and access to a third machine on another account enrolled as "family" on that subscription. There are folders shared between these accounts, in both directions. I also have folders shared with me from another account (to which I don't have direct access).
About 9 months ago the shares from the "unrelated" account became .url's when (as it now seems) that account was migrated. About 2 months ago the "family" shares went the same way, when apparently the "family" account migrated. About 2 weeks ago my own account finally also migrated, and the OneDrive client on both affected machines "processed" some 200,000 "changes" over about a day (which proved quite disruptive, even though no actual data was transferred, as file synchronisation was interrupted). All relevant accounts have therefore now been migrated.
After a further couple of days, the shares from the "unrelated" account did indeed revert to a folder in File Explorer, albeit with a name appended with the name of one of the affected computers, and all files were re-downloaded correctly. All the "family" shares, however, remain as links - in one case there are two links, one with the original name, the other appended with a computer name (both link to the correct folder online, with the original name).
So, two weeks on from the migration, I wonder if continuing to wait is still appropriate, or whether it is time to try removing and re-creating the links online?
And the correct answer was... continue to wait. Another 4 days on (so nearly 3 weeks from the final account migration) and the second set of links has at last reverted to folders, with no intervention from me. So over 9 months, but all now back working, leaving just one folder needing renaming to remove the appended computer name.