digikam 7.0.0-beta1 Dec 31 2019 for Windows 10 64 bit Thumbnails for *.heic files are not shown in Timeline view
Actually, I restarted the application and the thumbnails were generated properly. False alarm, I guess.
Press F5 to force to rebuild thumbnail in current album (virtual or not). Probably the database was busy. Don't forget that sqlite can be a bottle neck with large request (as search queries) Gilles Caulier