You need to remove the space in between the two clips. Right-click on the green space, then select “remove”. You are seeing black because your playhead is positioned on the blank space. Hope this helps.
I was about to mention speed change does not work well across all formats. You might consider to use the Properties > Convert to Edit-friendly function in Shotcut.
I have an el-cheapo Panasonic camera that uses AVCHD, but I can choose to change it to MP4 in the settings. It does come with really crappy software to convert AVCHD to MP4, and internally it will convert the AVCHD to MP4 if you transfer from within the camera via USB cable. I would go to your camera’s manufacturers support site, and I’d bet you’ll find conversion software there to download.
Unfortunately I can’t change the settings to MP4 in my camera. I have a Sony and the software they provide is a joke. It continually crashes and takes forever to transfer files.
My only option that makes sense is to export the AVCHD files to my Mac via the memory card. Then I need to convert them to MP4, but the trick is to select the correct file when I export them. I think I’ve got that covered now.