Rename Move does not work when only case is changed

[pinggit]

I found very often (not sure everytime) that, when I change a filename from "Abc-Def.txt" to "abc-def.txt", neither Rename nor Move works. is this expected?
I'm with Vim 8.1 on wsl, if that matters.

[tpope]

Can confirm I'm seeing this too on macOS. Vim's built-in :saveas has the same problem—calling :saveas abc-def.txt keeps the name Abc-Def.txt—and that makes finding a solution tricky.