Just in case if this haven't been solved yet or have approached another issue... I'll just leave a reply here..
I think there's really a bug that can happen on your preview window, but if it also gets bugged out in the export, maybe you need to check the FPS on both preview and in the export settings.
About the still image being present while a transition is happening, is it a slowed down clip? It really does happen sometimes if your clip goes lower than 50% to 25% of speed.
I can only read the conversation you have here. Sometimes there is a case where the files from the camera itself was the one to make it act strange. Also the speed. Note that slowing down a clip can make a clip jumpy as like it just hit a shockwave in slow motion. If my math is correct...
X*Y
We'll substitute Y as 0.15 since Nationalsolo mentioned.
If your main file runs at 100FPS, that'll result in a 15 frames per second at 15% speed. You won't feel anything becoming a still image for this one.
At 50FPS, it will be 7.5 frames per second. This is where you'll feel that there was some still images.
At 30FPS, the problem with still frames might occur, as it only shows 4.5 frames per second on a 15% clip speed. 9 frames on a span of 2 seconds to be honest.
I hope this bit of info can help you solve the problem.
Also, I wish to look at your project file, but I cant. One of the clips was 2GB in size, and I had a data limit that I need to save, so I can't look at your file.
Also, if there was some moments that a millisecond blank view occurs, check if there were no gaps on the timeline. I have some problems on those when I got multiple layers of video on my timeline, and have like a millisecond between splits.