Steps to reproduce:
1. Open any file for editing
No need to make actual changes to the file
2. Split the window using the Vim command ":vs"
3. Change to focus back to the original split using "Cmd-W Cmd-W", or clicking in the split with the mouse
4. Try to save the file using the Vim command ":w"

Result:
The field in the lower-right corner displays the error message: "File not saved".

NB: Using File -> Save "file.cpp" does work fine, though, which means that this is not an issue with permissions.

The issue persists even after closing one of the splits.

This feature works fine with QtCreator 4.14.x.