The bug appears in this configuration:
Client 2.4.11 on Windows 10 x64 (last updates installed)
Server 2.4.14 on Debian 9 (last updates installed)
Looks like some changes were made to the code earlier, but they still don’t work (I reported the bug a year ago).
Files with incorrect names are not visible by network access (if the backup folder shared using Samba).
In Midnight Commander, the filename displayed will contain a diamond with a question mark inside. The program hints that one UTF-8 character is corrupted.
Criticality of the bug: I think “high”. This makes it impossible to completely copy the archive over the network to another computer. Files with corrupted names will not be copied.
It will definitely be a very bad surprise.