I can reproduce it with the current master.
My guess: Must have sth. to do with /include/tool/Editing/HTML.php, $required_attr array and BuildAttributes() method
Must be a recent change. Works with 5.0 initial release.
edit:/ Likely in this commit
edit2:/ If one provides an alt attribute in CKEditor, the image survives hence the missing required alt attribute messes up the img tag.
Ok, all fixed. Thanks for putting me on the right track Juergen