I wrote about changing picture/slide duration in this post: Can't change duration of a slide - #4 by Hudson555x
I just tried all of the various ways to change the length and with version 18.08.14 they all work fine.
There has been a few releases this month, and you might be running 18.08.14, but it’s helpful for others to know, especially dev’s, to know the exact version you’re running.
The newest version is: 18.08.14 at the time of this posting, and can be downloaded here: Shotcut - Download
With 18.08.14, I get the same, although I have never tried changing the resolution at this location.