I'm copying from local PC source with 2400 folders to Backblaze bucket using S3 protocol, with drag & drop in the Commander GUI interface. Once I reach 1999 folders in the bucket, any extra folder that I transfer means the earliest folder transferred is no longer shown in the destination window. I never see more than 1999 folders, and if I transfer a file then that doesn't appear in Commander view either, but it does successfully transfer. The status bar on the remote pane also shows only 1999 items.
If I try the Synchronize function, WinSCP reports the earliest folders as missing. But if I access the bucket directly via a web browser separately from WinSCP then I can see all folders and files (approx 2400). I couldn't see this issue mentioned in doco or forum posts, so thanks for reviewing.
WinSCP 5.19.2 on Windows 10 Home (fully patched). S3 protocol (no compression) using the Commander interface. I haven't tried earlier versions. Log file attached.