SUMMARY
Moving, copying, or linking a file or directory results in Dolphin attempting to overwrite the destination directory if the destination directory is a symbolic link, and the link and linked-to directory are on different filesystems.

STEPS TO REPRODUCE
1. With two filesystems that support symbolic linking, create a directory 'a' somewhere on one file system.
2. Create a symbolic link 'a-link', pointing to 'a' and placed somewhere on the other file system.
3. Open the 'a-link' in Dolphin.
4. Using Dolphin, attempt to move, copy, or link a file or folder from anywhere into 'a-link'. You may do this by click and drag or by copy and paste.

OBSERVED RESULT
A dialog appears:
> File Already Exists -- Dolphin
> Warning, the destination is more recent.
> Source <the file being moved, copied, or linked>
> Destination <'a-link' or whatever the actual instance is called>
> Rename: [...], [Suggest new name] [Rename] [Overwrite] [Cancel]

Renaming will cause the copied file to be placed (with the new name) in the same directory as 'a-link' rather than in 'a-link'. Overwriting will cause the copied file to overwrite 'a-link'.

EXPECTED RESULT

The file/directory is moved, copied, or linked (respectively) into 'a-link'.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (rolling)
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.14.2

ADDITIONAL INFORMATION

I was able to reproduce this variously between ext4, ntfs, and ecryptfs. Using the linked-to directory (i.e., 'a') in Dolphin results in entirely normal behavior. Similarly, if the link and linked-to directory are on the same filesystem, there is no problem. It also doesn't seem to matter what file system the file/directory being moved, copied, or linked resides on.

This behavior seems to be new since version 19.12.3

Making the same copy via the system shell works as expected.