I am having the same issue with the Fade Out Video filter. I commented on this before. I discovered this problem while placing cross-dissolves.
Apparently, it is not resolved yet. I am now running Shotcut 18.03.06 on Linux KDE Neon 5.12 with a Ryzen 3 and 16 gb ram. The Fade Out Video displays either no fade or using the Throbber it may display an incomplete fade. The only way I have found to fix this is to remove the Fade Out Video filter and select it again - until it works properly.
Note: you may not notice the problem if the slideshow is not being composited over a colored background track. -=Ken=-