I may be too optimistic too early — again — but I have not restarted my phone since Monday when I updated to 17.4 and WhatsApp has done daily backups since, but my storage is stable, no increase until now. This may actually be fixed.
You might be right. I expected 17.4 to fix the problem, but after the first backup my storage did actually increase. However, the second backup stabilized the storage. I disabled the daily backup due to the bug, but I’ll enable it again now and see what happens