Good find! I have seen a problem like this years ago with metadata embedded in the media files, and there was a fix for it. In this case, this is bad text data is coming from the text box in the Properties panel of Shotcut. I will have to add some filtering there now to prevent this problem. Instead of deleting the whole <producer>...</producer> block, you could have simply deleted the lines with the offending characters. Thank you for finding it and reporting it.
Hmm, that worries me. Did you view the log or open these .mlt files to look for a similar problem? I will review all the changes from 18.08.01 to 18.08.11 to see if there is a possible reason, but it will be easier to find steps to reproduce.