Because it is a bug or glitch of the SP&R filter. Like I said above, I remember reading about this on another thread, but I can’t find it anymore.
Also, if I remember correctly, the black bars while playing the transition is also related to aspect ratio.
Your images don’t have the same aspect ratio as your project.
This gives me an idea… Maybe you don’t need to crop the top/bottom of your taller images. Please wait a few minutes while I do some tests…
Will do - I just had a idea myself, so have my own test to run… more on it soon
Edit: Didn’t work, tried to use the SP&R filter on the larger image, and copy the size position onto the smaller image to try make them the same “size”, but when you try to adjust one position, another changes also, not making it possible to cheat it that way.
Ok, here’s my new proposition.
Your images all need to have the same ratio.
Since cropping is not a good solution for you, there is still a way to make them all have the same ratio.
Solution: Imbed each of your image in a transparent frame that will have the same aspect ratio as your project.
For that you’ll need Photoshop, or Gimp. There are also online tools you can use, like PhotoPea.
Now that all your images have the same size as your project, you wont get that black bars problem anymore. That means you won’t need the SP&R filter anymore on the track head.
There is still a small problem though. The blurred part of the Bar Horizontal transition is semi-transparent, revealing the pink color from the track below. Maybe this is not a big deal for you. If it is, reducing the blur level (Softness) of the transition helps.
I will certainly attempt this another day (tomorrow), it is late at night once again,and maybe one day soon I will be able to begin my video, I hope!!
As for the transition, I am not too bothered by that if this solves the issue of the aspect ratio.
Will post again tomorrow with the results, thanks for the help as always 
No problem. It took a long time to get to this point. I wish I had thought of the Photoshop solution earlier.
Good night
Keep it simple.
All the contortions and gymnastics with Gimp and Photoshop are unnecessary.
One Filter To Rule Them All. Alpha Channel: Adjust.
The simple solution to your dilemma is here.
bonne nuit…
Ah! This must be the thread (or one of the threads) that I vaguely remember seeing about this problem.
Works well.
I think this is exactly the workaround that should apply to the bug I reported a while ago and is still not fixed: Clip transition applies to lower layers
Sadly adding the alpha channel filter at 0%, 50%, 100% or anywhere in between the ‘amount’ setting to both images doesn’t seem to remove the background coming through the transition, but maybe I am doing it wrong.
@ineedhelp
The Alpha Channel: Adjust solution is just an easier method to use instead of my Photoshop suggestion.
It gets rid of the black background problem that occurs during the transition.
I wish it would work for me - I can’t fathom why it does not… do you have the same setup as I do? Following all the same steps we have done so far with the same settings? Might be something there that is causing me to not be able to do what you can do
Also the black background is no longer a issue - it is the images changing sizes rather than remaining constant like in the video example I gave of what I am trying to imitate.
Did you remove the SP&R filter from the track head? It is no longer needed.
I have the same effect you have, yet it is not what is in the video example of what effect I need achieved, I have made a video example to illustrate better, of the video I am imitating with the correct transition, and then mine:
Also I do have the SP&R filter on, but I still have the pink background coming through… somewhat annoying, but not the biggest issue right now
Even without out it, it doesn’t make a difference for me - have no idea why…
One thing I should have asked in the first place is the original images YOU use.
If you could share them here, we would be on the same level.
The background image and 2 or 3 of your other images should be enough.
Don’t put them here separately, instead make a ZIP of all the images and share the ZIP file here.
Should have done it earlier to be honest, but better late than never - the background image and several samples, of different sizes:
Thanks you. I hope this will help.
However, I won’t have time to test them now. I’m leaving soon for a Easter family diner.
Meanwhile, maybe someone else will jump into this discussion an come up with the perfect solution 
Have a good time, I will spending the rest of the night messing around and hoping I get lucky - it worked once before, so you never know, it might happen again 
Thank you.
By the way, what is the Video Mode (Size and frame rate) of your Shotcut project ?
Your background image (thumbnail.png) size is 1569x883. Is your project set to these dimensions as well?
To find out you project’s resolution, select Output in the Timeline look in the Properties tab